Method and apparatus for aligning a cassette

ABSTRACT

An alignment tool, method and system are provided for aligning a cassette handler to a robot blade in a workpiece handling system, in which the tool comprises a frame or fixture adapted to be supported by the cassette handler support surface, in which the frame has one or more distance sensors positioned to measure the distance of a workpiece or robot blade from the sensor or a predetermined reference point or surface. In a preferred embodiment, the frame emulates a workpiece cassette and the distance sensors provide a numerical output of the distance to the workpiece. As explained in greater detail below, these distance measurements facilitate accurately leveling and aligning the cassette handler support surface relative to a workpiece supported by the robot blade such that when the frame is replaced by an actual workpiece cassette, the workpiece cassette will also be level and aligned with respect to the robot blade and the workpiece held by the blade. As a consequence, accidental scratching and breakage of workpieces such as semiconductor wafers and display substrates may be reduced or eliminated.

FIELD OF THE INVENTION

[0001] The present invention relates to automated workpiece handlingsystems, and more particularly, to methods and devices for aligning acassette for workpieces in an automated workpiece handling system.

BACKGROUND OF THE INVENTION

[0002] In order to decrease contamination and to enhance throughput,semiconductor processing systems often utilize one or more robots totransfer semiconductor wafers, substrates and other workpieces between anumber of different vacuum chambers which perform a variety of tasks. Anarticle entitled “Dry Etching Systems: Gearing Up for Larger Wafers”, inthe October, 1985 issue of Semiconductor International magazine, pages48-60, describes a four-chamber dry etching system in which a robothoused in a pentagonal-shaped mainframe serves four plasma etchingchambers and a loadlock chamber mounted on the robot housing. In orderto increase throughput, it has been proposed to utilize two loadlockchambers as described in U.S. Pat. No. 5,186,718. In such a two loadlocksystem, both loadlock chambers are loaded with full cassettes ofunprocessed wafers. FIG. 1 of the present application illustrates twotypical loadlock chambers LLA and LLB, each having a cassette 190therein for holding unprocessed wafers 192 to be unloaded by a robot 194and transferred to various processing chambers 196 attached to amainframe 198.

[0003] The loadlock chamber LLA, for example, is a pressure-tightenclosure which is coupled to the periphery of the mainframe 198 byinterlocking seals which permit the loadlock chamber to be removed andreattached to the mainframe as needed. The cassette 190 is loaded intothe loadlock chamber LLA through a rear door which is closed in apressure-tight seal. The wafers are transferred between the mainframe198 and the loadlock chamber LLA through a passageway 199 which may beclosed by a slit valve to isolate the loadlock chamber volume from themainframe volume.

[0004] As shown in FIG. 2, a typical cassette 190 is supported by aplatform 200 of a cassette handler system 208 which includes an elevator210 which elevates the platform 200 and the cassette 190. The platform200 has a top surface which defines a base plane 220 on which thecassette 190 rests. As the cassette includes a plurality of “slots” 204or wafer support locations, the elevator moves the cassette tosequentially position each of the slots with the slit valves to allow arobot blade to pass from the mainframe, through the slit valve, and to alocation to “pick” or deposit a wafer in a wafer slot.

[0005] The slots 204 of the cassette may be initially loaded with asmany as 25 or more unprocessed wafers or other workpieces before thecassette is loaded into the loadlock chamber LLA . After the loadlockaccess door is closed and sealed, the loadlock chamber is then pumped bya pump system down to the vacuum level of the mainframe 198 before theslit valve is opened. The robot 194 which is mounted in the mainframe198 then unloads the wafers from the cassette one at a time,transferring each wafer in turn to the first processing chamber. Therobot 194 includes a robot hand or blade 206 which is moved underneaththe wafer to be unloaded. The robot 194 then “lifts” the wafer from thewafer slot supports supporting the wafers in the cassette 190. By“lifting,” it is meant that either the robot blade 206 is elevated orthe cassette 190 is lowered by the handler mechanism 208 such that thewafer is lifted off the cassette wafer supports. The wafer may then bewithdrawn from the cassette 190 through the passageway and transferredto the first processing chamber.

[0006] Once a wafer has completed its processing in the first processingchamber, that wafer is transferred to the next processing chamber (orback to a cassette) and the robot 194 unloads another wafer from thecassette 190 and transfers it to the first processing chamber. When awafer has completed all the processing steps of the wafer processingsystem, and two cassettes full of wafers are loaded in the loadlocks,the robot 194 returns the processed wafer back to the cassette 190 fromwhich it came. Once all the wafers have been processed and returned tothe cassette 190, the cassette in the loadlock chamber is removed andanother full cassette of unprocessed wafers is reloaded. Alternatively,a loaded cassette may be placed in one loadlock, and an empty one in theother loadlock. Wafers are thus moved from the full cassette, processed,and then loaded into the (initially) empty cassette in the otherloadlock. Once the initially empty cassette is full, the initially fullcassette will be empty. The full “processed” cassette is exchanged for afull cassette of unprocessed wafers, and these are then picked from thecassette, processed, and returned to the other cassette. The movementsof the robot 194 and the cassette handler 208 are controlled by anoperator system controller 222 (FIG. 1) which is often implemented witha programmed workstation.

[0007] As shown in FIGS. 2 and 3, the wafers are typically very closelyspaced in many wafer cassettes. For example, the spacing between theupper surface of a wafer carried on a moving blade and the lower surfaceof an adjacent wafer in the cassette may be as small as 0.050 inches.Thus, the wafer blade must be very thin, to fit between wafers ascassettes are loaded or unloaded. As a consequence, it is oftenimportant in many processing systems for the cassette and the cassettehandler 208 to be precisely aligned with respect to the robot blade andwafer to avoid accidental contact between either the robot blade or thewafer carried by the blade and the walls of the cassette or with otherwafers held within the cassette.

[0008] However, typical prior methods for aligning the handler andcassette to the robot blade have generally been relatively imprecise,often relying upon subjective visual inspections of the clearancesbetween the various surfaces. Some tools have been developed to assistthe operator in making the necessary alignments. These tools haveincluded special wafers, bars or reference “pucks” which are placed uponthe robot blade and are then carefully moved into special slotted orpocketed receptacles which are positioned to represent the tolerancelimits for the blade motions. However, many of these tools have a numberof drawbacks. For example, some tools rely upon contact between theblade or a tool on the blade and the receptacle to indicate a conditionof nonalignment. Such contact can be very detrimental to high precisionmechanisms for moving the blade as well as the blade itself. Also, manysuch tools do not indicate the degree of alignment or nonalignment butmerely a “go/no-go” indication of whether contact is likely.

[0009] In aligning the handler mechanism to the robot blade, oneprocedure attempts to orient the cassette to be as level as possiblewith respect to the robot blade. One tool that has been developed toassist in the leveling procedure has dual bubble levels in which onebubble level is placed on the blade and the other is placed on thecassette. The operator then attempts to match the level orientation ofthe blade to that of the cassette. In addition to being very subjective,such bubble tools have also often been difficult to see in the closeconfines of the cassette and handler mechanisms.

[0010] As a consequence of these and other deficiencies of the prioralignment procedures and tools, alignments have often tended to be notonly imprecise but also inconsistent from application to application.These problems have frequently lead to the breakage or scratching ofvery expensive wafers and equipment as well as the generation ofdamaging particulates in the systems.

SUMMARY OF THE INVENTIONS

[0011] The present inventions are, in one aspect, directed to analignment tool, method and system for aligning a cassette handler to arobot blade in a workpiece handling system, in which the tool comprisesa frame or fixture adapted to be supported by the cassette handlersupport surface, in which the frame has one or more distance sensorspositioned to measure the distance of a workpiece or robot blade fromthe sensor or a predetermined reference point or surface. In a preferredembodiment, the frame emulates a workpiece cassette and the distancesensors provide a numerical output of the distance to the workpiece. Asexplained in greater detail below, these distance measurementsfacilitate accurately leveling the cassette handler support surfacerelative to a workpiece supported by the robot blade such that when theframe is replaced by an actual workpiece cassette, the workpiececassette will also be level with respect to the robot blade and theworkpiece held by the blade. As a consequence, accidental scratching andbreakage of workpieces such as semiconductor wafers and displaysubstrates may be reduced or eliminated.

[0012] In another aspect of the present inventions, the output of thedistance sensor or sensors may be used to determine the height of theworkpiece held by the blade relative to a predetermined reference pointor surface. This reference point is related to the actual measurementsof a production wafer cassette. As a result, the workpiece cassetteelevator of the cassette handler system may be set to accuratelyposition the robot blade and workpiece at preferred heights for varioushandler operations such as the slot base and slot delta positions (whichare a function of the space or distance between adjacent slots in agiven cassette) of the workpiece cassette being emulated, for example.

[0013] In yet another aspect of the present inventions, the distancesensors may be used to map the path of a workpiece as it is moved in orout of the frame. The data collected may then be displayed in agraphical or other format to represent the path of the workpiece androbot blade through a volume of space. This volume may be compared to apreferred volume of space (e.g., the envelope of a production wafercassette) and the path of the blade and workpiece, and adjusted toensure that the path remains within the preferred volume of space.

[0014] In still another aspect of the present inventions, the frame hasa predetermined reference surface positioned opposite the distancesensors of the frame. In a preferred embodiment, the frame referencesurface is accurately positioned by the frame to be at a predeterminedorientation and distance from a cassette handler reference point orsurface such as the cassette handler support surface. As a consequence,as explained in greater detail below, distance measurements to theworkpiece or robot blade may be output as offsets from thispredetermined frame reference surface which significantly facilitatescalibrating the distance sensors.

[0015] In a further aspect of the present inventions, the frame has analignment surface which may be aligned with a corresponding alignmentsurface on the robot blade to align the robot blade to the frameemulating the workpiece cassette. In a preferred embodiment, the frameand the robot blade each have an alignment aperture which receives analignment pin when the frame and robot blade are aligned in a selectedoperational orientation such as a wafer pickup position. Robot controlvariables such as blade rotation and extension step counts may then beset to define a blade position at the desired aligned position.

[0016] In still another aspect of the present inventions, the frame hasa plurality of sets of registration surfaces which are each adapted toregister the frame to the cassette handler support surface. As aconsequence, the frame may be seated in the cassette handler in aplurality of orientations. As explained below, such an arrangementfacilitates performing alignment and height setting operations at widelyspaced blade height positions to increase the accuracy of thoseprocedures.

[0017] In yet another aspect of the present inventions, a preferredembodiment includes a computer operated graphical user interface whichcan significantly facilitate rapid and accurate performance of alignmentand setting procedures utilizing the alignment frame of the presentinventions. As set forth below, the computer assisted embodiments canperform various calculations including preferred blade height positionssuch as the slot base and slot delta positions and elevatorcharacterizations for example. Actual measurements may be compared topreferred values calculated or otherwise provided and appropriateadjustments made.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments of the invention are described with reference to theaccompanying drawings which, for illustrative purposes, are schematicand not drawn to scale.

[0019]FIG. 1 is a schematic top view of a typical deposition chamberhaving two loadlock chambers.

[0020]FIG. 2 is a schematic front view of a typical wafer cassettedisposed on a platform of a cassette handling system.

[0021]FIG. 3 is a partial view of the wafer cassette of FIG. 2,depicting a wafer resting in a slot and a wafer picked up from a slot.

[0022]FIG. 3A is an enlarged partial view of the wafer cassette of FIG.3, depicting a wafer resting in a slot and a wafer picked up from aslot.

[0023]FIG. 4 is a schematic pictorial view of a cassette alignment toolsystem in accordance with a preferred embodiment of the presentinvention.

[0024]FIG. 5 is a side view of the metrology cassette of FIG. 4.

[0025]FIGS. 6A, 6B and 6C are a schematic partial cross-sectional topviews of the metrology cassette of FIG. 5, showing distance sensors invarious configurations.

[0026]FIG. 7 is a schematic view of display of the interface controllerof the system of FIG. 4.

[0027]FIG. 7A is a view of the computer display of FIG. 4, depicting aninput-output screen used in a calibration procedure.

[0028]FIG. 8 is a front view of the metrology cassette of FIG. 4.

[0029]FIG. 9 is a view of the computer display of FIG. 4, depicting aninput-output screen used in a leveling procedure.

[0030]FIG. 10 is an enlarged view of a portion of the screen of FIG. 9,graphically depicting the leveling inputs for a typical cassette handlerplatform.

[0031]FIG. 11 is a schematic view of the display of the interfacecontroller of the system of FIG. 4, during a wafer height measurementprocedure.

[0032]FIG. 12a is a top view of the metrology cassette of FIG. 4illustrating an extension and rotation alignment procedure.

[0033]FIG. 12b is a side view of the metrology cassette of FIG. 4illustrating an extension and rotation alignment procedure.

[0034]FIG. 12c is a front view of the metrology cassette of FIG. 4illustrating insertion of an alignment pin during an extension androtation alignment procedure.

[0035]FIG. 12d is a top view of the metrology cassette of FIG. 4illustrating insertion of an alignment pin during an extension androtation alignment procedure.

[0036]FIG. 13 is a view of the computer display of FIG. 4, depicting aninput-output screen used in an extension and rotation alignmentprocedure.

[0037]FIG. 14a is a partial side view of the metrology cassette of FIG.4 illustrating insertion of an alignment pin during an extension androtation alignment procedure when the cassette is in an invertedposition.

[0038]FIG. 14b is a front view of the metrology cassette of FIG. 4illustrating insertion of an alignment pin during an extension androtation alignment procedure when the cassette is in an invertedposition.

[0039]FIG. 15 is a view of the computer display of FIG. 4, depicting aninput-output screen used in height measurement procedure for a slotdelta position.

[0040]FIG. 16 is a front view of the metrology cassette of FIG. 4,showing the metrology cassette in an inverted position.

[0041]FIG. 16a is a partial schematic view of a wafer resting on thereference surface of the metrology cassette in an inverted position.

[0042]FIG. 17 is a view of the computer display of FIG. 4, depicting aninput-output screen used in a blade motion mapping procedure.

[0043]FIG. 18 depicts a manual worksheet used in a leadscrewcharacterization procedure.

[0044]FIG. 18a is a view of the computer display of FIG. 4, depicting analternative input-output screen used in a leadscrew characterizationprocedure.

[0045]FIG. 19 is a view of the computer display of FIG. 4, depicting aninput-output screen used to input wafer cassette specifications.

[0046]FIG. 20 is a view of the computer display of FIG. 4, depicting aninput-output screen used to input wafer dimensions.

[0047]FIG. 21 is a view of the computer display of FIG. 4, depicting aninput-output screen used to input robot blade dimensions.

[0048]FIG. 22 is a schematic diagram of the interface controller signalprocessing circuit for sampling signals from the laser head sensors.

[0049]FIG. 23 is an enlarged schematic diagram illustrating the effectof wafer edge curvature on the position of a wafer supported in acassette.

[0050]FIG. 24a is an enlarged schematic side cross-sectional viewillustrating the effect of wafer edge support on the position of a wafersupported in a cassette.

[0051]FIG. 24b is an enlarged schematic top view illustrating the effectof wafer edge support on the position of a wafer supported in acassette.

DETAILED DESCRIPTION

[0052] A cassette alignment tool system in accordance with a preferredembodiment of the present invention is indicated generally at 400 inFIG. 4. The cassette alignment tool 400 comprises a metrology cassette410, cassette controller 412 coupled by a communication cable 414 to themetrology cassette 410, and a computer 416 coupled by a communicationcable 418 to the cassette controller 412. The metrology cassette 410 issecured to the cassette handler platform 200 in the same manner as anactual wafer cassette such as the cassette 190 of FIG. 2 and thusemulates the wafer cassette 190. For example, the metrology cassette hasalignment and registration surfaces including an H-bar 430 and siderails 570 which are received by the cassette handler to align thecassette with respect to the handler. In addition, the metrologycassette 410 approximates the size and weight of a production wafercassette full of wafers.

[0053] The cassette alignment tool system 400 may be used withprocessing systems having one or many processing chambers and one ormore workpiece handling systems for transferring workpieces from one ormore cassettes in one or more loadlock chambers to one or more of theprocessing chambers. Once a particular handling system has been properlyaligned and calibrated to the robot blade and workpiece, the metrologycassette 410 may be removed from the handler and processing ofworkpieces may begin using a standard workpiece cassette which wasemulated by the metrology cassette 410. However, it is preferred thatall handlers of a particular processing system be properly aligned priorto initiating processing of production workpieces.

[0054] In accordance with one aspect of the illustrated embodiments, themetrology cassette 410 has a distance measurement device 500 which canprovide precise measurements of the position of a wafer or otherworkpiece being held by the robot blade within the metrology cassette410. As explained in greater detail below, these wafer positionmeasurements can be used to accurately align an actual wafer cassettesuch as the cassette 190 to the robot blade in such a manner as toreduce or eliminate accidental contact between the blade or the waferheld by the blade and the cassette or wafers held within the wafercassette.

[0055] As best seen in FIGS. 5 and 6, in the illustrated embodiment, thedistance measurement device 500 of the illustrated embodiment includesthree laser sensors A, B and C, each of which includes a laser head 510b, 510 r or 510 y, which is clamped in a mounting 512 b, 512 r or 512 y,respectively, carried by the metrology cassette 410. The mountings 512b, 512 r and 512 y are preferably color coded and mechanically keyed toreduce or eliminate inadvertent exchanges or misplacements of the laserheads in the mountings. Thus, the mountings 512 b, 512 r and 512 y maybe color coded blue, red and yellow, respectively, for example.

[0056] In the illustrated embodiment, the distance sensors are lasersensors manufactured by NaiS/Matshshita/Panasonic (Japan), modelANR12821 (high power) or ANRL 1821 (low power). This particular lasersensor operates based upon perpendicular beam, scattered reflectiontriangulation using a position sensing diode array. The light source(laser) impinges upon the target perpendicular to the surface of thetarget, preferably within a relatively small angle. The surfacepreferably provides a diffuse reflection that is visible to the sensingdevice over a relatively wide angle. The field of view of the sensingdevice is focused upon a linear optical sensor, the output of which isinterpreted to determine the displacement of the target surface withinthe field of view. The geometry of the light path therefore forms aright triangle with light from the light source traveling along thevertical edge and reflected light of the return path traveling along thediagonal. The distance between the sensor and the target may then becalculated using the Pythagorean theorem.

[0057] Although the distance sensors are described in the illustratedembodiments as three laser sensors, it is appreciated that other typesand numbers of distance measuring sensors may be used. For example,there are several different techniques and methods utilized bycommercial laser distance sensors. These include scattered lighttriangulation, reflective triangulation, perpendicular and angled beamtriangulation, time delta, interference pattern deciphering, CCD arraysensors, position sensing diode sensors, position sensing photoresistorsensors, etc. It is anticipated that a variety of non-laser andnon-light based distance measuring sensors may be suitable as well.

[0058] In the embodiment of FIG. 6A, the heads 510 b, 510 r and 510 y ofthe laser sensors are positioned in an equilateral triangular placementwhich facilitates a three point plane distance determination formeasuring the height of a surface such as a wafer surface. As explainedin greater detail below, the laser heads may be readily repositioned toother placements including an in-line placement for blade motion mapping(FIG. 6B), and a modified right triangle placement (FIG. 6C) foron-blade measurements.

[0059] Sensor Calibration

[0060] In another aspect of the illustrated embodiments, the metrologycassette 410 includes a precision internal reference surface 520 (FIG.5) which provides a fixed reference point from which all measurementsmay be gauged. It is fixed at the top of the cassette whereas the lasersensors are fixed to the bottom. The laser sensor light beams 522 areintercepted by the reference surface 520 when no wafer is present insidethe metrology cassette 410 and are reflected by the surface 520 back tothe laser heads of the laser sensor.

[0061] In the illustrated embodiment, the metrology cassette 410 ismanufactured so that the reference surface 520 is relatively flat andparallel with respect to the base plane 220 of the platform 200 of thecassette handler to a relatively high degree of precision. Allsubsequent distance measurements of the wafer can be made as offsets tothis reference surface 520. Because of the effects of temperature andaging of electronics, the output of the laser sensors can often varyover time. Thus, the actual value of the laser measurements of thedistance D_(REF) between the laser sensors and the reference surface 520can also vary over time even though the actual distance remains fixed.However, because all subsequent distance measurements of the wafer aremade as offsets to this reference surface 520, whatever value the lasersdetermine the distance D_(REF) between the laser sensors and thereference surface 520 to be, that value is considered to be the “zero”distance. Any subsequent measurement of wafer position is calculated asthe difference or offset D_(OFF) between the measured reference distanceD_(REF) and the measured wafer distance D_(WAF). Hence, calibrating thelaser sensors is simply a matter of turning the laser sensors on andafter a sufficient warm up time, noting the measured reference distanceD_(REF) and assigning that value as the “zero” distance.

[0062] For example, in the illustrated embodiment, once the cassettealignment tool system 400 has powered up properly the operator will seethree (3) red laser light spots on the reference surface 520. For somelaser_sensors it may take up to five seconds for the laser spots toappear. As the laser heads warm up, the distance values displayed foreach laser head by the interface controller display 530 (FIG. 7) mayfluctuate. To ensure adequate warm up time for the displayed values tostabilize, the interface controller 412 may include a built-in timerwhich displays a warm-up timing bar at the bottom of a display 530 whichmay be an LCD display for example. Other types of displays may be usedincluding the display 540 of the computer 416 which may display agraphical user interface (GUI). The warm-up timing bar on the bottomline of the display 530 may be programmed to disappear when the laserheads have warmed up (typically in about five (5) minutes).

[0063] When the warm-up is complete the bottom line will display***WARMUP COMPLETED***”. At this time, the interface controller display530 will display the raw distance values next to “blue”, “yellow” and“red” labels for each laser's output. As explained in greater detailbelow, the outputs of the metrology cassette 410 laser sensors aresampled and averaged over a period of time sufficient to substantiallycancel out noise and vibration effects.

[0064] The operator may now “zero”, or calibrate the cassette alignmenttool system 400 by pressing a button 532 on the interface controller412, which is labeled “ZERO”. In response, the system assigns the threedistance values measured by the three laser heads to be the distanceD_(REF) between that laser sensor and the reference surface 520 for eachlaser head. In that this distance value for each laser is the “zero”distance, the displayed measurement values for each laser head, labeled“blue”, “yellow” and “red”, are set to indicate 0.000 as shown in FIG.7. Calibration of the laser sensors is thus completed in a simple mannerwithout requiring any extemal instruments or tools.

[0065]FIG. 7A shows an example of an input-output screen 700 of agraphical user interface of the display 540 of the computer 416 that mayalso be used to calibrate the distance sensors. The screen 700 has a“button” 702 labeled “zero without wafer” which may be activated by theoperator moving the display cursor over the button 702 and depressingthe appropriate mouse or other input device button. Again, in response,the system assigns the three distance values measured by the three laserheads to be the distance D_(REF) between that laser sensor and thereference surface 520 for each laser head. The three distance values dA,dB and dC for the three laser heads A, B and C, respectively, are eachassigned an output value of 0.0000 inches as shown in the screen of FIG.7A.

[0066] Although the reference surface 520 of the metrology cassette 410of the illustrated embodiment is described as being flat and parallel,it is appreciated that other shapes and orientations of referencesurfaces and points may be used, depending upon the application. Also,the computer 416 is illustrated as a standard “laptop” size computer. Avariety of computing devices may be used including workstations anddedicated processors. The computer 416 preferably has memory includingshort term and mass storage memory as well as processors andinput-output devices including keyboards, printers, display screens andmouse or other pointing devices. The computer 416 is preferablyprogrammed to facilitate the implementation of the procedures discussedherein.

[0067] Workpiece Target Surface Calibration

[0068] In accordance with another aspect of the present embodiments, itis recognized that targets being sensed by distance sensors may respondto the distance sensors in different manners. For example, in theillustrated embodiment, laser sensors are used to measure the distancefrom the sensor to the reference surface 520 and also to measure thedistance to a workpiece, which is a silicon wafer in the illustratedembodiment. These sensors operate on the principle of the target havinga surface which reflects a light wave emitted by the sensor, back to thesensor. The sensors of the illustrated embodiment emit laser beams inthe red visible range. However, a small portion of the emission is inthe near-infrared range, and silicon wafers have a degree oftransparency to infrared radiation. As a consequence, the infraredportion of the radiation from the laser sensors is typically notreflected by the outermost exterior surface of the silicon wafer but isusually reflected at an internal depth within the silicon wafer. Bycomparison, the reference surface 520 of the illustrated embodiment hasa treated surface which preferably reflects the sensor beam more closelyfrom the actual exterior of the reference surface.

[0069] Because the reference surface and the workpiece may responddifferently to the sensor beams from the sensors, an error or deviationmay be introduced into the measurements of the true distances. Theresponsiveness of the target surface of the workpiece and the targetsurface of the reference surface 520 may be measured and compared todetermine any such difference which may be expressed as a correctionfactor. This correction factor may then be applied to distancemeasurements of the target workpiece to compensate for the manner inwhich the target workpiece responds to the sensor beams and therebyreduce or eliminate any such error caused by such differences.

[0070] To determine the correction factor, the distance sensors arefirst calibrated in the manner discussed above with no wafer present inthe metrology cassette. Thus, the “button” 702 labeled “zero withoutwafer” of the screen 700 may be activated by the operator moving thedisplay cursor over the button 702 and depressing the appropriate mouseor other input device button. Accordingly, the laser beams emitted bythe laser sensors and reflected by the reference surface 520 are sensedto provide the reference distances D_(REF) to the reference surface foreach laser head.

[0071] The metrology cassette 410 may then be inverted and placed on asuitable supporting surface. In this position, a wafer 230 a may beconveniently positioned on and supported by the metrology cassettereference surface 520. In this position, the laser sensing beams arereflected by the wafer 230 a rather than the reference surface 520. Ifthe laser beams are reflected by the exterior surface of the wafer, thedistance measurement D_(TGT) to the target would change by the thicknessW_(THICK) of the wafer. However, because silicon wafers have a degree oftransparency to infrared radiation, the measurement of the distance fromthe sensors to the wafer provides a measurement value D_(TGT) whichdiffers from the previously measured reference distance D_(REF) to thereference surface 520 by a value which is less than the thickness of thewafer as shown in FIG. 16a. By comparing this difference value(D_(REF)−D_(TGT)) to the known thickness W_(THICK) of the wafer, thecorrection factor F_(COR) may be calculated asF_(COR)=W_(THICK)−(D_(REF)−D_(TGT)). Thus, upon activating a “calibratetarget wafer” button 704 (FIG. 7A), the distance D_(TGT) from thesensors to the wafer is noted for each laser head such as laser head 510r and used with the previously measured reference distance D_(REF) tothe reference surface 520 and the known wafer thickness W_(THICK) tocalculate the correction factor F_(COR) for each laser head. Subsequentmeasurements of the distance to the wafer may then be corrected bysubtracting the correction factor F_(COR) from the measured distancevalue D_(TGT) to provide the corrected distance D_(WAF) which is a moreaccurate representation of the distance from a laser head sensor to theouter surface of the wafer.

[0072] Because the response of a target such as a silicon wafer to adistance sensor such as a laser sensor may vary from wafer to wafer, itis preferred that the same wafer be used for subsequent aligning andcalibration procedures discussed below. It should also be appreciatedthat correction factors may be determined for other types of targets andsensors, correcting for the variations in the manner in which particulartargets respond to particular sensors. In addition to placing the targetwafer on the reference surface 520 for target surface calibration whenthe metrology cassette is placed in the inverted position, the targetmay also be affixed to the reference surface by an appropriate mechanismwhen the metrology cassette is in the noninverted position.

[0073] Cassette Handler Leveling

[0074] In aligning a wafer cassette to a robot blade, it is preferredthat the wafer cassette be arranged so that wafers stacked within thecassette are as parallel as possible to a wafer held within the pocketof the robot blade when inserted into the cassette. The parameteraffecting this is the alignment of the blade to the cassette slots,which are provided by thin flat or angled shelves or teeth 1912extending outward from the sidewall of the cassette, and designed tohold the wafers parallel to the base of the cassette. Accordingly,cassette handlers typically have various adjustment mechanisms on theplatform 200 of the cassette handler which adjusts the forward/backwardand left/right tilt of the platform so that the base of the cassettesecured to the platform, and thus the shelves upon which the wafer sits,are oriented parallel to the robot blade. These forward/backward andleft/right adjustments to the platform are typically referred to as“leveling” the cassette handler although achieving a true horizontalleveling is typically not the goal.

[0075] As explained below, a cassette alignment tool system 400 inaccordance with a preferred embodiment of the present invention readilypermits the cassette handler to be “leveled” relative to the wafer bladeboth quickly and very accurately. Instead of relying upon visualestimates or the mechanical contact tools of prior methods, the cassettealignment tool system 400 of the illustrated embodiments accuratelymeasures the left/right and forward/back displacements of a robot bladecarrying a wafer relative to the reference plane 520 of the metrologycassette 410 and provides a numerical output indicating both thedirection and amount of each displacement. Using this information, theoperator can readily adjust cassette handler until the system 400indicates that the amount of left/right and front/back displacements arezero or within tolerance. The following provides an example of such acassette handler leveling operation for a typical loadlock chamberdesignated “LLA”.

[0076] First, the operator causes the robot to extend the robot bladeinto loadlock “LLA ” to the “drop position” and so that the operator canplace a clean wafer in the blade pocket. To facilitate light beamreflection by the wafer, it is preferred that the mirror side of thewafer be placed face up, with the dull silver side down to face thelaser sensors. The robot blade is then retracted back in the transferchamber to the zero position, with the wafer properly in the robot bladepocket. The metrology cassette 410 of the cassette alignment tool system400 is then placed on the loadlock “LLA ” cassette handler platform inthe same manner as a standard plastic cassette. Using the systemcontroller, the loadlock “LLA ” cassette handler moves the metrologycassette 410 to “slot base 24”. The “slot base” position is the cassetteposition relative to the robot blade in which the blade is preferablymidway between two wafers resting in consecutive slots. For example,FIG. 2 illustrates the slot base 25 position for wafer cassette 190which is the vertical position of the wafer cassette 190 when the robotblade 206 is midway between two wafers 230 and 232 in resting inconsecutive slots 24 and 25, respectively, of the wafer cassette 190.The metrology cassette 410 of the illustrated embodiment does not haveactual slots for supporting wafers. However, in that the metrologycassette 410 is emulating the wafer cassette 190, the positions of thewafer slots formed between adjacent shelves for a production cassettecan be readily supplied from the cassette manufacturer, in terms of adistance offset relative to the reference plane 520. Thus, for thisleveling procedure, FIG. 8 shows the effective slot base 24 position forthe metrology cassette 410 when the robot blade 206 is midway betweentwo imaginary wafers 234′ and 232′ resting in consecutive imaginaryslots 23 and 24, respectively, of the metrology cassette 410 190. Theoperator may visually check the location of the metrology cassette 410and the cassette handler to ensure that it is at “slot base 24” for loadlock “LLA”. The cassette alignment tool system 400 may then becalibrated by pushing the Zero button on the cassette alignment toolsystem 400 controller as described above to ensure that “L/R” and “F/B”displayed values on the display 530 of the interface controller are bothreading 0.0000 as shown in FIG. 7. The L/R displayed value is thedifference between the distance measurements of the blue and yellowlaser heads 510 b and 510 y, respectively, which are disposed on theleft and right, respectively as shown in FIG. 6A. The F/B reading is thedifference between the averaged distance measurement of the blue andyellow laser heads 510 b and 510 y, respectively, which are disposed inthe front of the metrology cassette 410, and the red laser head 510 rwhich is disposed in the back of the metrology cassette 410 as shown inFIG. 6A. Because the robot blade and wafer have not yet been extendedinto the metrology cassette 410, the light beams of the laser distancesensors will intercept the cassette reference surface 520. As previouslymentioned, the distance measurements of the three lasers to the flat,parallel reference surface 520 during the “zeroing” operation arecalibrated to be output as zero. Thus, the difference between the leftand right laser distance measurements is assigned an L/R output of zeroand the difference between the front and back laser distancemeasurements is assigned an F/B output of zero.

[0077] Following calibration of the lasers, the robot blade and wafermay be extended into the cassette alignment tool system 400 metrologycassette 410 preferably making sure there is no contact from the robotblade and wafer with any part of the cassette alignment tool system 400metrology cassette 410. The robot blade and wafer may be stopped at the“wafer drop” position which is the position at which the blade drops awafer into a slot or picks a wafer up from a slot. Transfer robotmovements are typically commanded through a processing systemcontroller.

[0078] After the robot blade is moved into the cassette, the distanceD_(WAF) (FIG. 8) from each laser sensor to the bottom surface of thewafer on the robot blade is measured by the three sensors. Afterallowing a couple of seconds for the reading on the display 530 of theinterface controller to stabilize, the outputs labeled “L/R” and “F/B”may be noted. The offset distances D_(OFF) from the reference surface520 to the wafer (D_(REF)−D_(WAF)) may then be displayed for each laserhead as shown in FIG. 11. In the example of FIG. 11, the offset distanceD_(OFF) for each laser sensor is displayed as 1.333 which will be thesame for each sensor if the robot blade is properly leveled relative tothe cassette reference surface 520. Since the L/R displayed value is thedifference between the distance measurements of the blue and yellowlaser heads 510 b and 510 y, respectively, which are disposed on theleft and right, respectively as shown in FIG. 6A, the L/R displayedvalue will be 0.0000 if the cassette is properly leveled in theleft-right direction. Similar, because the F/B reading is the differencebetween the averaged distance measurement of the blue and yellow laserheads 510 b and 510 y, respectively, which are disposed in the front ofthe metrology cassette 410, and the red laser head 510 r which isdisposed in the back of the metrology cassette 410 as shown in FIG. 6A,the F/B displayed value will be 0.0000 if the cassette is properlyleveled in the front-back direction. Thus, if the cassette is leveled tothe robot blade both readings will be 0.0000. If not, the cassette willneed to be leveled relative to the robot blade.

[0079] The cassette handler of the illustrated embodiment has threeleveling screws which may be individually adjusted to change thefront/back and left/right orientation of the platform 200, and thus thecassette to the robot blade. These leveling screws are graphicallyrepresented in a convenient computer display output 800 shown in FIG. 9,the relevant portion of which is shown in an enlarged view in FIG. 10.As shown therein, the three leveling screws are labeled #1, #2 and #3,respectively.

[0080] The following provides an example of use of a cassette alignmenttool in accordance with an embodiment of the present invention forleveling a cassette handler. Of course, the procedure may be readilymodified to accommodate the particular leveling adjustment mechanism ofthe particular handler being used.

[0081] First, the operator levels the handler in the front to back (F/B)direction by adjusting the slotted screw labeled #1 about ¼ of a turnclockwise (CC) for example, and allowing the F/B measurement displayedby the interface controller display 530 (FIG. 11) to stabilize after thechange. If the F/B reading becomes a smaller value (closer to the0.000), the operator should continue to adjust the #1 screw until theF/B becomes 0.000. If the display F/B value becomes larger, the operatorcan turn the #1 screw counterclockwise (CCW). It is preferred that theoperator make small adjustments, waiting for the display reading tostabilize before the operator makes the next adjustment.

[0082] Next, the handler may be leveled in the left to right (L/R)direction by adjusting the slotted screw labeled #3 using the samemethod of adjustment described above. The operator preferably should notneed to adjust slotted screw #2 unless the operator cannot level thecassette within the desired tolerance such as 0.0020, for example, inboth the F/B and L/R directions. When both of the F/B and L/R readingsare 0.0020 or better, the cassette platform is level to the robot blade.

[0083] As previously mentioned, the metrology cassette 410 is emulatingthe wafer cassette 190. In that the dimensions of the blade, wafer andwafer cassette are known or can be measured, a preferred slot baseposition can be calculated for each slot base of the cassette 190. Sucha preferred slot base position for slot base 24 is represented as aheight H_(sb) (FIG. 8) above the plane of the base plane 220 of theplatform 200. Similarly, the calculated preferred slot base position mayrepresented as an offset distance D_(sb) from the cassette referencesurface 520.

[0084] To facilitate leveling the cassette relative to the robot blade,the laser distance measurements by the laser sensors to the underside ofthe wafer held by the robot blade relative to the reference surface 520when the robot blade is inserted into the cassette may be output to theoperator as displacements from the calculated preferred slot baseposition D_(sb) measured from the cassette reference surface 520.

[0085]FIG. 9 shows the computer display screen 800 having an outputlabeled “Blade Left/Right (A−B)” which is similar to the L/R output ofthe interface controller display discussed above. However, the displayedvalue “A−B” is the difference between the two displacements, onemeasured by the blue (left) laser 510 b and one measured by the yellow(right) laser 510 y, from the calculated preferred slot base positionD_(sb). Another output labeled “Blade Front/Rear (avg AB)−C)” is similarto the F/B output of the interface controller display but the value “C”is the displacement measured by the red (back) laser 510 r, from thecalculated preferred slot base position D_(sb). If the cassette isleveled to the robot blade, both readings will be 0.0000 because thedisplacements from the preferred slot base position will be zero foreach laser, indicating a level condition. If not, the operator can levelthe cassette to the robot blade in the same manner described above,adjusting the leveling screws until the desired 0.0000 readings (orwithin tolerance) are obtained.

[0086] Robot Blade Extension and Rotation Alignment

[0087] In addition to leveling the cassette to the wafer blade, it isalso very useful to properly set the “wafer drop” or “wafer pick”position of the wafer blade relative to the cassette. As set forthabove, the “wafer drop” position is usually the same as the “wafer pick”position and is the position at which the blade drops a wafer into aslot or picks a wafer up from a slot. In many processing systems, thetransfer robot can move the wafer blade in a rotational movementcentered about a pivot point 199 (FIG. 1) on the robot shoulder. Inaddition, the blade can be extended radially outward and withdrawnradially inward in a translational movement. These movements commandedthrough the processing system controller are typically defined in termsof a rotation count and an extension step count. Each extension steprepresents an incremental translation movement of the robot blade andeach rotation count represents an incremental rotational movement of theblade. The system controller can cause the blade to rotate and thenextend or to both rotate and extend in combined motions in response torotation step commands and extension step commands inputted to thesystem controller by the operator.

[0088] In accordance with another aspect of the illustrated embodiments,the metrology cassette 410 has an alignment hole 600 (FIG. 12a) in thetop plate 612 which permits an alignment plug 614 (FIG. 12b) to beinserted through the cassette top plate alignment hole 600 and through asimilar alignment hole 616 in the robot blade 206 when the robot bladeis properly positioned in the drop/pickup position as shown in FIG. 12c.Furthermore, the cassette alignment tool system 400 can provide agraphical operator interface which facilitates the blade extension androtation alignment procedure.

[0089] As used herein, the term blade refers to the wafer blade 206illustrated and discussed as well as other robot hands for holding awafer or other semiconductor workpiece such as a display panelsubstrate, which is loaded and unloaded from a cassette in asemiconductor processing system.

[0090] Referring now to FIG. 9, the operator may select the“Extension/Rotation” button 810 of the Leveling Display 800 by movingthe computer display cursor to the Extension/Rotation button 810 andclicking the left mouse button. A Blade Extension and Rotation worksheet820 will pop up on the computer display screen, as shown in FIG. 13.

[0091] To ready the cassette alignment tool system 400 for the rotationand extension alignment procedure, the metrology cassette 410 is placedon the cassette handler of a loadlock such as loadlock “LLA” with thetop plate 612 up as shown in FIG. 12b and the cassette alignmentsurfaces such as the H-bar 430 of the bottom plate 630 properlyregistered with the handler alignment surfaces of the platform 200. Theoperator then causes the processing system controller to move theloadlock “LLA” cassette handler to slot base # 24 and then extend therobot blade to the Drop Position/Pick Position of loadlock “LLA ” asshown in the side view of FIG. 12b. The operator may then insert theExtension/Rotation Alignment Plug 614 into the alignment hole 600 in thetop plate to determine if the barrel end 615 of the alignment plug 614is aligned with an alignment hole 616 in the robot blade. If the robotblade alignment hole 616 is properly aligned with end 615 of thealignment plug 614 and hence the cassette alignment hole 600, the end615 of the alignment plug 614 will pass through the blade alignment hole616 as shown in the cassette front view of FIG. 12c and the cassette topview of FIG. 12d. In the illustrated embodiment, the alignment hole 600and the blade alignment hole 616, each has a diameter of ⅛″ but may ofcourse have other dimensions and placements, depending upon theapplication. Furthermore, the alignment surfaces of the metrologycassette, the alignment plug and the robot blade may have a variety ofshapes and positions other than the cylindrical shapes illustrated.

[0092] To align to the blade alignment hole 616 to the alignment plugend 615, the operator may command the processing system controller tomake small adjustments in the current settings of the blade extensioncount to extend or withdraw the blade, and in the blade rotation countto rotate the blade either clockwise or counter-clockwise as needed.When the operator has adjusted the robot blade to the properExtension/Rotation position, the alignment plug end 615 should dropthrough the alignment hole 616 in the robot blade easily with no help orforce from the operator.

[0093] The operator can record both the readings of the “Blade ExtensionStep Count” and the “Blade Rotation Step Count” on the Blade ExtensionRotation worksheet 820 (FIG. 13) in windows provided for that purpose.As an example, the value 17080 has been entered in an upper left window822 for the “Extension Step Count”. Similarly, the value −5880 has beenentered in an upper right window 824 for the “Rotation Step Count.”

[0094] In accordance with another aspect of the illustrated embodiments,the top plate 612 of the metrology cassette 400 has cassette alignmentand registration surfaces including an H-bar 622 in the same manner asthe bottom plate 630 which permits the metrology cassette 400 to beinverted so that the top plate 612 engages and aligns to the handlerplatform 200 as shown in FIG. 14a. As a consequence, the alignment hole600 in the plate 612 of the metrology cassette 400 may be used to alignthe robot blade rotation and extension positions when the blade is in asubstantially lower slot base position such as slot base #2.

[0095] Accordingly, after inverting and reseating the metrology cassette400 as shown in FIG. 14a, the operator causes the processing systemcontroller to move the loadlock “LLA” cassette handler to slot base # 24and then extend the robot blade to the Drop Position/Pick Position ofloadlock “LLA”. The operator may then insert the Extension/RotationAlignment Plug 614 into the robot blade alignment hole 616 as shown inFIG. 14b to determine if the barrel end of the alignment plug 614 isaligned with an alignment hole 600 of the cassette plate 612. If thealignment blade alignment hole 616 is properly aligned with the cassettealignment hole 600, the end 615 of the alignment plug 614 will passthrough the plate alignment hole 600. Again, when the operator hasadjusted the robot blade to the proper Extension/Rotation position, thealignment plug end 615 should drop through the alignment hole 600 in thecassette plate easily with no help or force from the operator.

[0096] The operator can record both the readings of the “Blade ExtensionStep Count” and the “Blade Rotation Step Count” for slot base #2 on theBlade Extension Rotation worksheet 820 (FIG. 13) in windows provided forthat purpose. As an example, the value 17100 has been entered in a lowerleft window 826 for the “Extension Step Count”. Similarly, the value−5890 has been entered in a lower right window 828 for the “RotationStep Count.”

[0097] The values of the rotation and extension step counts for eitheror both of the slot base positions may be entered into the processingsystem controller for controlling the movements of the robot blade toset the blade extension and rotation counts for the blade dropoff/pickupposition for the loadlock chamber. Alternatively, and in accordance withanother aspect of the illustrated embodiments, the cassette alignmenttool system 400 can automatically calculate and display an average ofthe Extension Step Count from both readings taken at slot base #24 andslot base #2, respectively, and also an average of the Rotation StepCount from both readings taken at slot base #24 and slot base #2,respectively. In the illustrated embodiment, these averages aredisplayed as bold numbers at the bottom of the Blade Extension andRotation screen 820 in the boxes 830 and 832, labeled Calculated IdealEXTENSION Count and Calculated Ideal ROTATION Count, respectively. Forexample, FIG. 13 shows a Calculated Ideal EXTENSION Count number to be17090 and a Calculated Ideal ROTATION Count number to be −5885. Thesecalculated average values may be input into the processing systemcontroller for controlling the movements of the robot blade to set theblade extension and rotation counts for the blade dropoff/pickupposition for the loadlock chamber.

[0098] The metrology cassette 410 may be used with a variety of robots,robot blades, elevators, system controllers and cassettes other thanthose depicted and described to align and set a variety ofblade/cassette positions other than those described.

[0099] Height Alignment

[0100] In accordance with another aspect of the illustrated embodiments,the cassette alignment tool system 400 provides a convenient means tomeasure the height of a wafer on the robot blade at various positionsrelative to the loadlock cassette platform. These measurements can beused to ensure, for example, that the robot blade is at the “slot base”and “slot delta” heights appropriate for the particular wafer cassette.The system may also be used to verify and correct other heights as well,depending upon the application.

[0101] As previously mentioned, the slot base height is the verticalposition of a wafer cassette when the robot blade 206 and the wafercarried by the blade 206 are midway between two wafers in consecutiveslots of the wafer cassette. FIGS. 3 and 3a illustrate the slot deltaheight which is the vertical offset of the wafer cassette above the slotbase position when a wafer such as the wafer 232 carried by the blade206 is centered in the slot.

[0102] As shown in FIG. 2, the platform has a physical home positionindicated at 250 which is the lowest point to which the elevator canlower the platform 220. Above the physical home position 250 is a“logical” home position 252 which is displaced from the physical homeposition 250 by a distance often referred to as the “home offset” whichis expressed in terms of the number of incremental steps which arenecessary for the elevator 210 to move the platform 220 from thephysical home position 250 to the logical home position 252. The numberof steps necessary for the elevator to move the platform a unit distance(expressed in English or metric units) is referred to as the “pitch” ofthe elevator. The logical home position expressed in terms of a stepcount may be assigned the step count “0” position. Above the logicalhome position is a position of the platform in which the cassettecarried by the platform is at one of the slot base positions. Forcassette 190, the bottom-most slot is slot #1. The platform positionwhich positions the cassette at a slot base position which correspondsto cassette position slot base #1 is indicated at 254 in FIG. 2. Thedistance between the bottom slot base position 254 and the logical homeposition is often referred to as the “bottom slot offset” (BSO) and isexpressed in terms of a step count.

[0103] To change the height of the robot blade to the height of aparticular slot base position, the cassette handler system is commandedto elevate the cassette so that the robot blade and the wafer carried bythe blade are at the desired slot base height relative to the cassette.If the BSO count is properly set into the cassette handler system, thecassette will be elevated to the appropriate height relative to therobot blade such that the robot blade and the wafer carried by the robotblade will be at the desired slot base height. If the BSO is notproperly set, the robot blade and the wafer carried by the blade maystrike an adjacent wafer or slot as the robot blade is moved inwardbetween two adjacent slots.

[0104] In accordance with another aspect of the illustrated embodiments,the height of a wafer carried by the blade relative to the cassette maybe accurately measured and compared to a preferred height for performinga particular operation. For example, as explained in greater detailbelow, a preferred slot base height may be calculated based upon thedimensions of the wafer cassette being emulated and the dimensions ofthe wafer. When the cassette handler system is commanded to elevate themetrology cassette relative to the robot blade which changes the heightof the cassette to a particular slot base height, the actual height ofthe blade relative to the metrology cassette may then be preciselymeasured and compared to the expected blade height or preferred slotbase height. Any difference between the measured and expected heightscan be determined as a numerical correction factor and appropriatecorrections may be made to the cassette handler system to ensure thatthe robot blade is at the preferred slot base height. In a similarmanner, the slot delta height can also be verified and corrected.

[0105] To measure the height of the robot blade and the wafer carried bythe blade, it is preferred that the laser sensors first be calibrated asset forth above. Thus, before the robot blade is moved into thecassette, the distance D_(REF) from each laser sensor to the referencesurface 520 of the metrology cassette is measured by each of the threesensors. In the illustrated embodiment, it is preferred that themeasured distances be displayed as offset distances from the referencesurface 520. Thus, after the “zero” button on the interface controlleris depressed, the measured distance value D_(REF) for each laser isoutput as zero as shown in FIG. 7. After the robot blade is moved intothe cassette, the distance D_(WAF) (FIG. 8) from each laser sensor tothe bottom surface of the wafer on the robot blade is measured by thethree sensors. The offset distance D_(OFF) from the reference surface520 to the wafer (D_(REF)−D_(WAF)) may then be displayed as shown inFIG. 11. In the example of FIG. 11, the offset distance D_(OFF) for eachlaser sensor is displayed as 1.333 which will be the same for eachsensor if the robot blade is properly leveled relative to the cassettereference surface 520 as discussed above. These measurements may becompared to expected offsets for a particular slot base position todetermine if the robot blade and the wafer carried by the blade areindeed at the desired slot base position. If not, the numericaldifference between the measured offset distances and the expected offsetdistances indicate both the amount and direction of the appropriatecorrections which can be made to the cassette handler system to ensurethat the blade and its wafer are moved to the desired slot base positionor other desired position.

[0106] In an alternative embodiment, the expected distance measurementswhen the blade and its wafer are at the preferred slot base position,may be inputted to the cassette alignment tool system or calculatedinternally by the cassette alignment tool system as discussed below.Thus, when the cassette alignment tool system 400 measures the blade andwafer height using the laser sensors, the output may be expressed interms of a displacement from the calculated preferred blade height forthat position. For example, if the cassette handler is commanded to movethe cassette to slot base #24, the measured blade position may bedisplayed as a displacement from the calculated preferred slot base #24position. FIG. 9 of the illustrated embodiment shows an example of sucha slot base #24 displacement having a value of 0.5005 as an average ofthe three measured displacements of the three laser sensors. If theplatform positions the cassette relative to the robot blade such thatthe robot blade is measured to be at the preferred slot base height, thedisplayed blade height value will be zero. If a nonzero blade heightmeasurement such as the 0.5005 value is displayed, the blade heightrelative to the cassette may be adjusted. In the illustrated embodiment,such adjustments are preferably made by modifying the bottom slot offsetstep count input to the cassette handler system.

[0107] The adjustment to the bottom slot offset count can beaccomplished empirically. That is, after determining the present bottomslot offset count input into the system, the operator can make aneducated guess based upon the magnitude and the sign of the displayedblade height displacement as to the amount of the count correction andthe direction of the count correction (either add or subtract) to modifythe bottom slot offset count setting. As set forth above, the height ofthe cassette relative to the robot blade at a particular slot baseposition may be modified by modifying the bottom slot offset (BSO)setting of the cassette handler system. After the operator modifies thebottom slot offset setting, the cassette handler system may be commandedagain to move the cassette to slot base #24 using the new BSO setting.The laser sensors will measure the blade position relative to themetrology cassette reference surface 520 and again, the cassettealignment tool system will display the measured displacement of theblade from the expected slot base #24 height. If necessary the BSO maybe corrected again and the cassette handler commanded to move the bladeto slot base #24 again. This process may be continued until thedisplayed blade height displacement value is “zero”, indicating that theheight of the blade is precisely at the calculated preferred height forslot base # 24.

[0108] In accordance with another aspect of the illustrated embodiment,the setting of the bottom slot offset value may be facilitated by thecassette alignment tool system by an offset position calculator whichcalculates an expected count value for an offset such as the bottom slotoffset. This expected count value is calculated based upon the distancethat the cassette handler elevator elevates the cassette for each stepand the measured displacement of the robot blade/wafer from the expectedposition. The number of steps per unit distance for the cassette handlerelevator may be a known quantity for a particular elevator.Alternatively, the step per distance value may be measured by thecassette alignment tool as discussed in greater detail below.

[0109]FIG. 9 shows an example of such an offset calculator at 850 in thedisplay screen 800. The calculator 850 labeled “BSO/Pickup OffsetCalculator” may be used as follows. After commanding the cassettehandler system to move the cassette to a slot base position such as slotbase #24, the laser sensors measure the wafer/blade position and outputthe measured displacement from the calculated preferred slot baseposition as discussed above. The operator may then enter the current BSOnumber (such as 46250, for example) into the “Current BSO Count” box 852on the system display screen 800 and clicking on the arrow button 854next to it. The operator may then read the predicted new BSO number fromthe result box 856 to the right.

[0110] The new BSO number (such as 32429, for example) is automaticallydetermined by the cassette alignment tool system 400 by multiplying thedisplayed blade height displacement (such as 0.5005, for example) by theelevator's steps per inch value. This product, either a positive ornegative step count value, indicates the number of steps which ispreferably added to (or subtracted from) the current BSO number toreposition the cassette at the preferred slot base height. This numbermay be entered into a system constants entry page (such as the systemtool page of FIG. 21) as the new BSO.

[0111] After the operator modifies the bottom slot offset setting, thecassette. handler system may be commanded again to move the cassette toslot base #24 using the new BSO setting of 185742. The laser sensorswill again measure the blade position relative to the metrology cassettereference surface 520 and again, the cassette alignment tool system willdisplay the measured displacement of the blade from the expected slotbase #24 height. If the BSO setting is correct, the displayed bladedisplacement value will be zero (or sufficiently small withintolerance). If necessary the BSO may be calculated again as describedabove, entering the “current” BSO value 185742 into the entry box 852and clicking the entry button 854 to obtain the new BSO value. Thisprocess may be continued until the displayed blade height displacementvalue is “zero”, indicating that the height of the blade is precisely atthe calculated preferred height for slot base # 24.

[0112] The blade/wafer height measurement process and apparatus of theillustrated embodiment has been discussed in connection with measuringthe blade/wafer height relative to the metrology cassette when thecassette handler has been commanded to move to a slot base position. Itshould be appreciated that the blade/wafer height may be measured atother positions as well. For example, the blade/wafer height may bemeasured at a slot delta position as shown by the computer input-outputscreen 1600 depicted in FIG. 15. In this example, the input-outputscreen 1600 indicates height measurements at the slot 24 delta position.Here too, the height of a wafer carried by the blade relative to thecassette may be accurately measured and compared to a preferred heightfor the slot delta position which may be calculated based upon thedimensions of the wafer cassette being emulated and the dimensions ofthe wafer, as explained in greater detail below.

[0113] After commanding the cassette handler to move the metrologycassette to the slot #24 delta position and calibrating the distancesensor as described above, the robot blade is moved into the metrologycassette, the distance D_(WAF) from each laser sensor to the bottomsurface of the wafer on the robot blade is measured by the threesensors. The offset distance D_(OFF) from the reference surface 520 tothe wafer (D_(REF)−D_(WAF)) may then be displayed as shown in FIG. 11.These measurements may be compared to expected offsets for a particularslot delta position to determine if the robot blade and the wafercarried by the blade are indeed at the desired slot delta position. Ifnot, the numerical difference between the measured offset distances andthe expected offset distances indicate both the amount and direction ofthe appropriate corrections which can be made to the cassette handlersystem to ensure that the blade and its wafer are moved to the desiredslot delta position or other desired position.

[0114] In the same manner as the slot base position, the expecteddistance measurements when the blade and its wafer are at the preferredslot delta position, may be inputted to the cassette alignment toolsystem or calculated internally by the cassette alignment tool system asdiscussed below. Thus, when the cassette alignment tool system 400measures the blade and wafer height using the laser sensors, the outputmay be expressed in terms of a displacement from the calculatedpreferred blade height for that slot delta position. For example, if thecassette handler is commanded to move the cassette to slot delta #24,the measured blade position may be displayed as a displacement from thecalculated preferred slot delta #24 position. FIG. 15 of the illustratedembodiment shows an example of such a slot delta #24 displacement havinga value of 0.3920 as an average of the three measured displacements ofthe three laser sensors. If the platform positions the cassette relativeto the robot blade such that the robot blade is measured to be at thepreferred slot delta height, the displayed blade height value will bezero. If a nonzero blade height measurement such as the 0.3920 value isdisplayed, the blade height relative to the cassette may be adjusted. Inthe illustrated embodiment, such adjustments to the slot delta heightare preferably made by modifying the pickup offset step count input tothe cassette handler system. The pickup offset step count is the offsetas measured in terms of the number of steps, that the slot deltaposition is above the slot base position for a particular slot.

[0115] The adjustment to the pickup offset count can be accomplishedempirically. That is, after determining the present pickup offset countinput into the system, the operator can make an educated guess basedupon the magnitude and the sign of the displayed blade heightdisplacement as to the amount of the count correction and the directionof the count correction (either add or subtract) to modify the pickupoffset count setting. As set forth above, the height of the cassetterelative to the robot blade at a particular slot delta position may bemodified by modifying the pickup offset (BSO) setting of the cassettehandler system. After the operator modifies the pickup offset setting,the cassette handler system may be commanded again to move the cassetteto slot delta #24 using the new Pickup offset setting. The laser sensorswill measure the blade position relative to the metrology cassettereference surface 520 and again, the cassette alignment tool system willdisplay the measured displacement of the blade from the expected slotdelta #24 height. If necessary the Pickup offset may be corrected againand the cassette handler commanded to move the blade to slot delta #24again. This process may be continued until the displayed blade heightdisplacement value is “zero”, indicating that the height of the blade isprecisely at the calculated preferred height for slot delta # 24.

[0116] In accordance with another aspect of the illustrated embodiment,the setting of the pickup offset value may be facilitated by thecassette alignment tool system by an offset position calculator whichcalculates an expected count value for an offset such as the pickupoffset in the same manner as the bottom slot offset calculator. Thus,here too the expected count value is calculated based upon the distancethat the cassette handler elevator elevates the cassette for each stepand the measured displacement of the robot blade/wafer from the expectedposition.

[0117]FIG. 15 shows an example of such an offset calculator at 800 inthe display screen 1600. The calculator 800 labeled “BSO/Pickup OffsetCalculator” may be used as follows. After commanding the cassettehandler system to move the cassette to a slot delta position such asslot delta #24, the laser sensors measure the wafer/blade position andoutput the measured displacement from the calculated preferred slotdelta position as discussed above. The operator may then enter thecurrent Pickup offset number (such as 3255, for example) into the“Current Pickup offset Count” box 1652 on the system display screen 1600and clicking on the arrow button 1654 next to it. The operator may thenread the predicted new Pickup offset number from the result box 1656 tothe right. The new Pickup offset number (such as 17076, for example) isautomatically determined by the cassette alignment tool system 400 bymultiplying the displayed blade height displacement (such as 0.3920, forexample) by the elevator's steps per inch value. This product, either apositive or negative step count value, indicates the number of stepswhich is preferably added to (or subtracted from) the current Pickupoffset number to reposition the cassette at the preferred slot deltaheight. This number may be entered into a system constants entry page(such as the system tool page of FIG. 21) as the new Pickup offset.After the operator modifies the pickup offset setting, the cassettehandler system may be commanded again to move the cassette to slot delta#24 using the new Pickup offset setting of 17076. The laser sensors willagain measure the blade position relative to the metrology cassettereference surface 520 and again, the cassette alignment tool system willdisplay the measured displacement of the blade from the expected slotdelta #24 height. If the Pickup offset setting is correct, the displayedblade displacement value ill be zero (or sufficiently small withintolerance). If necessary the Pickup offset may be calculated again asdescribed above, entering the “current” Pickup offset value 17076 intothe entry box 1652 and clicking the entry button 1654 to obtain the newPickup offset value. This process may be continued until the displayedblade height displacement value is “zero”, indicating that the height ofthe blade is precisely at the calculated preferred height for slot delta# 24.

[0118] Lower Slot Position Calibration

[0119] In accordance with yet another aspect of the illustratedembodiments, the metrology cassette may be inverted and replaced ontothe cassette handler platform to facilitate blade/wafer heightmeasurements at the lower slot number positions. For example, FIG. 16shows the metrology cassette 410 in the inverted position in which theprecision internal reference surface 520 which provides a fixedreference point from which all measurements may be gauged, is fixedadjacent the support 200, to emulate the bottom of a cassette whereasthe laser sensors are spaced from the support in a position at the topof a production cassette. In the illustrated embodiment, the plate 612and the associated registration surfaces of the metrology cassette 410are manufactured so that the reference surface 520 is relatively flatand parallel with respect to the base plane 220 of the platform 200 ofthe cassette handler to a relatively high degree of precision in theinverted position as well the noninverted position depicted in FIG. 5.The distance D_(INV) between the reference surface 520 in the invertedposition and the base plane 220 of the platform 200 is known. Again,like the noninverted position, all subsequent distance measurements ofthe wafer can be made as offsets to this reference surface 520. Thus,the wafer position in a position such as the slot #2 base may becalculated as the difference or offset D_(OFF) between the measuredreference distance D_(REF) and the measured wafer distance D_(WAF) asshown in FIG. 16. The wafer offset position D_(OFF) may be convertedinto a height measurement above the base plane 220 of the platform 200by adding the known distance D_(INV) to the measured wafer offsetposition D_(OFF). Such inversion is useful to measure the slot basepositions adjacent the support, in those applications in which thedistance sensors such as the illustrated laser heads would otherwiseinterfere with insertion or retraction of the wafer blade.

[0120] Blade Mapping

[0121] In accordance with another aspect of the illustrated embodiment,the cassette alignment tool system 400 provides a convenient means tomap the trajectory of a wafer on the robot blade (and by extension, theblade itself) to ensure proper alignment to the cassette. Such a mappedtrajectory may be displayed in an easy to understand graphical formatfor inspection by the operator as shown in the output screen 1800 (FIG.17) of the computer display 540. This motion analysis page 1800 may beaccessed using the “map blade motion” button 860 on the “leveling” page800 of FIG. 9.

[0122] The robot arms and the attached blade may exhibit a very complexmotion profile while moving into or out of a cassette. Thus, the robotarm of many wafer handling systems will typically exhibit a trajectoryrise or drop motion as the blade is inserted into or withdrawn from thecassette. Many blades also exhibit a sweep motion in which the bladesweeps, or rotates about some pivot point, in left and right rotationalmotions. Still further, blades can exhibit a twist motion which resultsin changes to the side-to-side level of the blade. Accordingly, it isdesired to determine the entire volume of space that is swept though bythe arm, blade and workpiece during their motion, to ensure that someportion of these components will not be in an interference position witha cassette component. Such a determination can be difficult to achieveby a visual inspection of the motion.

[0123] These motion components are interrelated, and have the net effectof making the wafer/blade combination appear larger than it actually is.If for example the blade droops at the arm wrist, but the arm motion isperfectly flat, the effective space occupied by the blade during itsmotions would have the same thickness as the blade/wafer combinationplus the amount of the droop. While an observer might detect the droop,the tendency of many operators might be to tilt the cassette to matchit, believing the blade followed the path of the droop. This correctionmay be insufficient if the motion of the blade in the droopedorientation is not taken into account.

[0124] Similarly a change in the twist orientation or a change in therobot arm height during the motion will also increase the effectivespace occupied by the blade and wafer. Even if the observer follows themotion through the cassette, the level of accuracy that can be achievedis often relatively low.

[0125] Furthermore, in many wafer handling systems, and particularly inprocessing systems that have been in service for a long period of time,the motions of the robot arm and the loadlock elevators are often notrepeatable to a high degree of precision. Withdrawing the robot blade toits zero position and extending it back out to the pickup or dropposition will typically result in slightly different measured height andleveling data each time the operation is performed. This is normal andis caused by the wear and backlash in the mechanical components. Systemshaving newer revisions of software that incorporate backlashcompensation can reduce these variations to a degree, but typically donot eliminate it entirely. As a result, programming movements of therobot blade to avoid contact with adjacent wafers and slots by observingthe path of the blade during one loading or unloading operation may notprevent contact with these obstacles in a subsequent operation due tothese variations.

[0126] In contrast, the cassette alignment tool system 400 of theillustrated embodiment is capable of precisely mapping many of thesemotions over one or more operations. As a result, a determination of theentire volume of space that is swept though by the arm and blade duringits various motions is readily facilitated by the laser sensors of themetrology cassette 410. Thus, the cassette alignment tool system 400 ofthe illustrated embodiment is capable of mapping many of these motionswith accuracy and repeatability utilizing the laser sensors of themetrology cassette. Moreover, the laser sensor data of this and eachprocedure of the cassette alignment tool may be readily recorded forlater reference. Further, the system can indicate when robot or elevatormaintenance is dictated, as where the sweep, droop and twist exceedallowable limits of the space between wafers, and it cannot becompensated by robot support 200 adjustment.

[0127] In accordance with one aspect of the illustrated embodiments, themapping functions of the cassette alignment tool system 400 can be usedto determine an outside envelope of the combination of motions,determine a mean (weighted average) path, and test these results againstpre-determined limit tolerances. Once this has been done, the data canbe used to suggest corrections to the operator.

[0128] Motion Envelope

[0129] As explained in greater detail below, the dimensions of thecurrent combination of cassette, blade and wafer when input into thesystem can be used to calculate a pair of vertical motion limits such asthe limits indicated at 1802 and 1804 in FIG. 17. These limits may becentered around a preferred centerline height 1806. These calculatedlimits may be overridden by means of a limits override check box 1808 atthe right hand side of the display window 1800. If the “fixed limits”entry box 1810 is checked, the value shown in the selection box 1812below is used to set the maximum robot blade motion tolerance window(MBMTW) instead of the calculated limits. For example, the number 30displayed in the selection box 1812 sets the total dimensional limit to0.030″, 0.0175″ above the 0.000 center line and 0.0175″ below the 0.000center line. The user can select other limits by choosing from thepull-down menu or entering another limit value. In this manner, a fixedmotion limit envelope may be selected to override the calculated limitsbased on the combination of hardware items used.

[0130] The vertical motion limits 1802 and 1804 may be thought of asdefining a horizontal slab of space into which all motions of the bladeand wafer preferably fit to avoid contacting the cassette slots or otherwafers. Pass-fail tests may then be applied to the resulting datagathered during a mapping operation.

[0131] As set forth above, the laser sensors 500 may be positioned in atriangular pattern to measure and define the plane of the wafer carriedby the blade in relation to the internal reference surface 520 builtinto the metrology cassette 410. To begin mapping the robot blademotion, the robot blade, with the wafer in the pocket, is preferablycommanded to the Pick/Drop position at slot base #24. This position maybe sampled and displayed by moving the operator's cursor to the buttonlabeled Sample and clicking the left mouse button, (or pushing thebutton 533 (FIG. 4) on the cassette controller labeled Select) and atriangle 1813 will appear on the graph grid at 0.000 and 0. This is theoperator's first sample and the data is recorded and displayed as theinitial “reference” sample 1813.

[0132] Subsequent samples are recorded as the robot arm and wafer arewithdrawn from the cassette in small increments as shown in FIG. 17.Each time a sample is taken, the measurement readings of the A (blue)and B (yellow) lasers (the two toward the opening of the cassette) aresampled, averaged, recorded and displayed.

[0133] To take the additional samples, the processing system may be setto permit the operator to manually step the robot blade from thePick/Drop Position to the Zero Position at a fixed number of steps at atime. That fixed number of steps is displayed below the graph above theSample button on the mapping display screen 1800 as shown in FIG. 17.For example, the blade may be stepped at 1250 steps per sample, eachstep resulting in a fixed distance movement of the blade. In thismanner, the operator can input the blade lateral distance movement basedon a constant distance per step, to produce the x-coordinate distancefor this mapping. Each time the operator moves the blade the fixednumber of steps, the operator may wait for the laser readings tostabilize (such as 5 or 10 seconds, for example) and then move theoperator's cursor to the button labeled Sample (Select) and click theleft mouse button, (or push the button on the cassette controllerlabeled Select) to record the operator's sample. The robot blade maythen be stepped again and sampled again towards the Zero Position.

[0134] The average path measured by the A (blue) and B (yellow) lasersis plotted, along with a projected C (the rear (red) sensor) for eachpoint sampled. In the graph of FIG. 17, each sampled data pointrepresenting the height measured by the A (blue) laser (the left laser)is marked on the graph using a blue downward pointing arrowhead 1830.Similarly, each sampled data point representing the height measured bythe B (yellow) laser (the right laser) is marked on the graph using ayellow upward pointing arrowhead 1832. In the example illustrated inFIG. 17, the left and right sides of the blade are initially level asindicated by the overlapping arrowheads 1830 and 1832 in the initialportion of the plotted path. However, the arrow heads 1830 and 1832progressively separate at the subsequent sample points indicating thatthe blade exhibits a twisting motion as it is withdrawn from thecassette.

[0135] In the illustrated embodiment, the C (red) laser beams no longerintercept the wafer once the blade and wafer begin to withdraw.Accordingly, extrapolated data rather than actual measurement data fromthe C laser is displayed.

[0136] When the operator has stepped the robot blade and the wafer outof the range of the last two lasers, the robot blade motion mappingsession will automatically end in the illustrated embodiment. A finalanalysis will update the Motion Analysis graph as shown in FIG. 17. Theoperator may then move the operator's cursor to the button labeledFinish and click the operator's left mouse button to indicate the end ofthe mapping operation.

[0137] The resulting graph depicted in FIG. 17 shows the motion of theblade and wafer. At completion, the highest and lowest points in themotion can be used to define the motion envelope. These may be any ofthe A, B, or extrapolated C laser data points. Nonetheless, the motionprofile may be examined as components of that motion. For example, thesystem can measure and analyze the height of a wafer carried by therobot blade relative to the metrology cassette as well as the degree oflevelness between the wafer and the metrology cassette as discussedabove.

[0138] The highest limit 1814 and the lowest limit 1816 of the motionenvelope are averaged to find a centerline 1820 for the motion. This isin turn may be compared to a preferred centerline 1806 as calculated.The difference is shown as a deviation. This deviation or offset fromthe calculated preferred centerline value is also reflected back anddisplayed as the measured average height on the leveling page 800 (FIG.9). The BSO value calculator on that page may be used to adjust the BSOvalue to cause the displayed offset value to become zero or close to itwhen retried. Upon rerunning the path mapping operation, the motionenvelope should be centered on the calculated preferred value. Further,if the envelope thickness exceeds the allowable wafer plus robot bladeallowance within the cassette, robot maintenance to correct the swing,twist, rotation, or other motion is indicated.

[0139] The cassette alignment tool system 400 also calculates a weightedmean path for the motion. This is an average path that represents theaverage of the entire path but compensated for the distance from thehorizontal center of the path. The reason this may be helpful is thatthere may be short vertical excursions (humps) in the path height thatwould otherwise be ignored. The computed rise or fall is also displayed.

[0140] After running a motion mapping, it is preferred that the bladeand wafer be returned to the Pickup or Drop position (extended) beforeattempting adjustments. This is because the displays on the Levelingpage 800 that are affected show real-time data. If the blade and waferare not present in the cassette, they may not be valid.

[0141] Elevator Characterization

[0142] In accordance with another aspect of the illustrated embodiment,the cassette alignment tool system 400 of the illustrated embodimentprovides an apparatus and a method for measuring the motion of theelevator of the cassette handler. These elevators often include aleadscrew or other mechanism for lifting and lowering the cassette.Typically, a leadscrew mechanism includes a threaded shaft, which iscoupled to a nut fixed against rotation and coupled to the cassettehandler platform. Rotation of the shaft causes linear motion of the nut,and thus of the handler. The leadscrew is commanded by the operatorsystem to elevate the cassette in steps. The actual linear distancemoved by the leadscrew is referred to as the leadscrew “pitch” and isexpressed in steps per inch (or steps per millimeter if metric). Theprecise value of the pitch for any one particular elevator may vary fromcassette handler to cassette handler. Thus, the pitch value provided bythe manufacturer may not provide sufficient accuracy because that valuemay not account for wear or for differing motor types and pulley ratios(of the pulleys connecting the motor to the shaft) that may be in use inthe field. Here, using the laser distance measuring sensors that arebuilt into the cassette alignment tool system metrology cassette 410,the operator can measure the height of a wafer on a blade at or nearopposing ends of the cassette elevator's useful travel such as at slotbases 2 and 24, for example. By dividing the change in step count by thechange in height, the pitch may be accurately determined. Because theelevator travel is measured over a relatively long “baseline” distance,errors are minimized.

[0143] In the illustrated embodiment, the operator measures the heightof the wafer and records the elevator step count in two positions, forexample, slot base 2 and slot base 24. FIG. 18 shows an entry sheet 1700which may be implemented as a manual worksheet or as an input screen forthe computer 416. To determine the height measured in inches of thewafer at the first position, e.g. slot base #24, the output readings ofthe three laser sensors are noted at 1702. FIG. 11 provides an exampleof three such output readings as displayed by the interface controller.The distance measurement output by each laser sensor corresponds to theoffset distance labeled D_(OFF) in FIG. 5 for each laser. As set forthabove, this offset distance is the distance of the wafer from thereference surface 520. If the blade and wafer are precisely leveled,each reading should be the same. If the readings differ slightly, suchas when the blade is leveled within acceptable tolerances, the threereadings may be averaged and the average noted at the entry box 1704.The distance between the base plane 220 of the cassette handler platform200 and the metrology cassette reference surface 520 in the noninvertedorientation is known and is indicated in FIG. 5 as D_(NOTINV). Hence,the height of the wafer above the base plane 220 of the cassette handlerplatform 200 at slot base #24 may be readily calculated asD_(NOTINV.)−D_(OFF) or D_(NOTINV)−1.333=H_(SB24) as indicated at 1705 inthe illustrated example. The current step count at slot base #24 shouldalso be noted in the space provided at 1706.

[0144] The robot blade may then be withdrawn from the metrology cassetteand the metrology cassette is then inverted as shown in FIG. 16. Aftercommanding the cassette handler to move the metrology cassette to slotbase #2, the robot blade may then be extended back into the metrologycassette and the wafer height determined and the current step countnoted as shown in FIG. 18 in the same manner as that done at slot base#24.

[0145] Thus, the output readings of the three laser sensors are noted at1732 in the input screen and the average noted at the entry box 1734.This distance measurement is the distance from the metrology cassettereference surface 520 to the top of the wafer in slot base #2. In theexample of FIG. 18, this distance is measured and averaged to be 1.300.

[0146] The distance between the base plane 220 of the cassette handlerplatform 200 and the metrology cassette reference surface 520 in theinverted orientation is known and is indicated in FIG. 16 as D_(INV.)Hence, the height of the wafer above the base plane 220 of the cassettehandler platform 200 at slot base #2 may be readily calculated asD_(INV.)+D_(OFF) or D_(IN)+1.300=H_(SB2) as indicated at in theillustrated example. The current step count at slot base #2 is noted inthe space provided at 1736.

[0147] The difference in heights and counts at the two positions is thenused to calculate the number of steps per inch or millimeter. Thus, forexample, if the elevator count at slot base 24 is 123456 and theelevator count at slot base 2 is 603155, the number of steps betweenslot base 24 and slot base 2 is 479699 (603155−123456) as indicated at1750. Similarly, the distance between the measured heights of slot base#24 and #2 is H_(SB24)−H_(SB2)=H_(SBF) inches as indicated at referencenumeral 1752 in FIG. 18. Because the laser sensors measured the distanceto the reference surface from the bottom of the wafer at slot base # 24and from the top of the wafer at slot base #2, the thickness of thewafer is preferably added to the measured height difference to provide amore accurate measurement as indicated at 1754 to provide a heightdifference measurement of H_(F) inches. The count difference betweenslot bases 24 and 2 may be divided by the corresponding heightdifference to provide the pitch P expressed in steps per inch asindicated at 1756. This pitch may be multiplied by the slot spacing perinch value to provide a steps per slot value S as indicated at 1758. Theslot spacing per inch value may be measured on the wafer cassette beingemulated or obtained from the wafer cassette manufacturer'sspecifications for the wafer cassette.

[0148]FIG. 18a shows an alternative entry screen 1780 for a graphicaluser interface for the computer display 416. In this embodiment, theoperator inputs the step count values at each of the two measured heightpositions, such as slot bases 24 and 2, in input boxes 1782 and 1784,respectively. In response, the computer can automatically compute anddisplay the elevator pitch and slot spacing values using the measuredheights at the two positions determined as set forth above.

[0149] In this manner, the steps per inch pitch and the steps per slotpitch values may be accurately determined for the elevator of the systemwhen used with the wafer cassette being emulated. It is preferred thatthe operator perform this elevator characterization procedure prior tousing the Bottom slot Offset (BSO) and Current Pickup Offset Countcalculators on the Leveling page to facilitate accurate setting of thesevalues.

[0150] Although the elevator of the illustrated embodiment utilizes aleadscrew mechanism, it is appreciated that the pitch and othercharacteristics of the elevator movement may be accurately determinedfor a variety of elevator mechanisms. In addition, utilizing distancesensors having an appropriate range, localized abnormalities may bedetected by taking multiple readings at spaced locations along theelevator travel path.

[0151] The cassette alignment tool system of the present invention maybe used with a variety of workpiece handling systems. For example, insome wafer handling systems the combinations of leadscrews, motors anddrive pulleys used are often the same. For these applications, astandard pitch value may be inputted into the cassette alignment toolsystem in lieu of an elevator characterization procedure. Should therebe nonetheless manufacturing variations, the elevator pitch may beaccurately determined as set forth above, or may be entered in numericalformat.

[0152] Once the desired calibration and alignment procedures discussedabove have been completed for a particular handling system and theassociated robot blade and workpiece, the metrology cassette 410 may beremoved from the handler and processing of workpieces may begin using astandard workpiece cassette which was emulated by the metrology cassette410. However, it is preferred that all handlers of a particularprocessing system be properly aligned prior to initiating processing ofproduction workpieces.

[0153] Metrology Cassette 410 Mechanical Construction and Features

[0154] The metrology cassette or fixture 410 of the illustratedembodiment is a precision frame assembly emulating the size and mountinginterfaces of a wide range of plastic wafer cassettes. The variableattributes of individual cassettes such as slot positions and spacingcan be defined in software instead of requiring physical changes to themetrology cassette 410.

[0155] As described above, the laser sensors housed within the metrologycassette 410 use the reference surface 520 of the cassette 410 as a“zero” point. In that the height of the reference surface 520 is known,the true height of the wafer may be easily calculated using the measuredoffset from the reference height. Since this height typically does notappreciably vary with time or temperature (normal extremes), the laserscan be “soft zeroed” using the offset measured from the referencesurface 520.

[0156] The laser sensors of the illustrated embodiment have a linearmeasurement range of 3.149Δ+/−0.7874Δ (80,00 mm+/−20,00 mm). Because ofthe thickness of the base plate 630 (FIG. 5) and the height of the laserhead mounting brackets 512 (FIGS. 5 and 6), the linear measurement rangeof the laser heads covers slots 1-4 and 22-25 for most styles ofcassettes. On some systems, the robot blade wrist may interfere with thetop and bottom plates, limiting the mechanically usable slot range to2-4 and 22-24. These ranges as well as other sizes, characteristics andvalues are provided as examples and can vary, depending upon the type ofdistance sensor selected and the intended application.

[0157] The laser head supports on the mounting brackets 512 may bepin-located and color coded in their positions, and are preferably notmechanically interchangeable so as to prevent setup errors. The laserheads may be located in a variety of patterns including the illustratedtriangular pattern (FIG. 6A) which facilitates height measurementoperations or an in-line pattern (FIG. 6B) which facilitates bladecharacterization. The particular pattern selected may vary dependingupon the application.

[0158] The mechanical framework of the metrology cassette 410 serves anumber of functions in addition to enclosing and supporting the lasersensors. One such function of the fixture is the precise positioning ofthe reference surface 520 for the laser sensors. It is preferably flat,parallel to the base, and precisely at a defined reference height. Inthe illustrated embodiment, this reference height of the referencesurface 520 is the height marked D_(NOTINV) which is the height of thereference surface above the cassette handler platform base plane 220when the metrology cassette is in the noninverted position as shown inFIGS. 5 and 9. It is preferred that this dimension be tightly controlledto increase the accuracy of the height measurements. The tolerancespecifications for this surface in the illustrated embodiment are asfollows: Flatness: +/−0.002″ (+/−0,05 mm) overall Parallelism: +/−0.002″(+/−0,05 mm) Height D_(NOTINV) (referenced to base plane 220 of platform200): D_(NOTINV) +/−0.002″ (181,04 mm +/−0,05 mm)

[0159] As set forth above, another preferred construction feature is thethickness of the upper reference plate 612 from its topmost surface inthe noninverted orientation to the reference surface 520. This thicknessdefines another reference height of the reference surface 520. Thissecond reference height is the height marked D_(iNV) which is the heightof the reference surface 520 above the cassette handler platform baseplane 220 when the metrology cassette is in the inverted position asshown in FIG. 16. Its specification in the illustrated embodiment is:

[0160] Thickness: D_(iNV)+/−0.002″ (+/−0,05 mm)

[0161] Adding the two reference heights to one another, the overallheight of the metrology cassette 410 is:

[0162] Total Height: D_(iNV)+D_(NOTINV)+/−0.004Δ (+/−0,10 mm)

[0163] Furthermore, the finish of the reference surface 420 ispreferably compatible with the laser sensors. In the illustratedembodiment, the reference surface 520 is lapped, ground and “vaporhoned” to a matte finish (0.000016Δ (0,00041 mm) RMS) to within+/−0.001Δ (+/−0,0255 mm) flatness across its entire working surface. Thereference surface is also hard anodized to deposit a layer whichprovides a surface which is similar to a white unglazed ceramic.

[0164]FIG. 12a shows a top view of the top plate 612 of the metrologycassette. The top plate 612 has base plane surfaces which engage thebase plane 220 of the cassette handler platform 200. Those cassette baseplane surfaces and other topmost surface features of the top plate arepreferably themselves flat within 0.002Δ (0,05 mm) for the fixture 410to fit into the system's cassette handler nest in the cassettenoninverted position without rocking. These features also may have tighttolerances applied to them so that the assembly will not have excessivelateral movements during its use. The cassette base plane surfaces andother surface features of the bottom plate 630 may be similarlyconstructed to facilitate fitting into the system's cassette handlernest in the cassette noninverted position.

[0165] As best seen in FIGS. 5 and 6, the metrology cassette 410 hasside rails 570 which support and locate the reference plate 612. Inaddition the side rails 570 maintain the “squareness” of the shape ofthe metrology cassette. A webbing 572 (FIG. 8) in the front (wafer entryside) of the fixture 410 is provided to increase its stability andstrength. These pieces also serve as registration surfaces for systemssuch as the P5000 Ergonomic Cassette Handler (sold by Applied Materials,Inc.) that rely upon certain upper-portion features for location.

[0166] In the illustrated embodiment the components of the fixture 410are preferably located and assembled with dowel pins 580 to ensure thatthe basic accuracy of the fixture is not compromised under normaloperating conditions. The top surface of the plate 612 and the bottomsurface of the plate 630 are both machined to imitate the bottomfeatures of common wafer cassettes. Thus, the exterior of the metrologycassettes emulates the bottom surface features, wafer cassette verticalprofile, sidebars, “H” bar, etceteras. This allows it to be insertedinto most systems with the reference plate on top or bottom. This isvery useful when characterizing leadscrews and determining slot spacing.In addition, this widens the applicability of the fixture because itallows upper and lower slot alignments to be performed. It also allowstopside and bottom side rotations and extensions to be determined. Thesefeatures include the H-bar 622 as shown in FIG. 12a. Variations andcompromises from the features of individual cassettes can be made so asto accommodate the widest possible range of systems and cassettes. Forexample, by choosing the smallest size of the registration surfaceswithin the permitted range of tolerances of the cassettes to beemulated, the number of cassettes which can be emulated by a single tool410 may be increased.

[0167] The metrology cassette 410 of the illustrated embodiment islightweight, preferably approximating the mass of a production wafercassette full of wafers. It should be noted that the precise location ofthe fixture in the horizontal plane (X-Y) is significant primarily inthe extension/rotation alignment setups because the plate 612 containsthe precision alignment hole 600 for extension and rotationdeterminations.

[0168] The dimensions, ranges, shapes, materials, sizes,characteristics, finishes, processes and values of the metrologycassette construction are provided as examples and can vary, dependingupon the intended application.

[0169] Calculation of Preferred Height Values

[0170] The following provides examples of equations and calculationsequences that can be used intemally by the cassette alignment toolsystem 400 to determine preferred heights such as those measured for theslot base and delta of a slot in the cassette, such as slot #25. Insituations where the top slot cannot be mechanically accessed, thedimensions are simply adjusted downward by the slot spacing dimension.

[0171] It is very useful to note that the laser sensors of theillustrated embodiment “see” the internal reference surface 520 of themetrology cassette 410 and the backside of a wafer on the blade. Thus,the following equations are, in the illustrated embodiment, not basedupon the mechanical centerlines of the blades, or the wafers on theblades.

[0172] Also, these calculations are based upon specifications of thewafer cassette being emulated. The values of these specifications may beobtained from the wafer cassette manufacturers and inputted manuallyinto a Cassette Specifications input screen 1900 shown in FIG. 19. Thecassette alignment tool system may also be programmed to provide thesevalues automatically in response to the operator inputting the model ofthe wafer cassette at an input box 1902 of the input screen 1900.

[0173] Similarly, these calculations are based upon dimensions of thewafers which are to be stored in the wafer cassettes. The values ofthese specifications, including wafer thickness and diameter, may beinputted manually into a Wafer input screen 2000 shown in FIG. 20. Thecassette alignment tool system may also be programmed to provide thesevalues automatically in response to the operator inputting the wafertype at an input box 2002 of the input screen 2000.

[0174] Further, these calculations are based upon dimensions of theoverall blade thickness and the blade pocket thickness of the robotblade used to carry the wafers into and out of the wafer cassettes. Thevalues of these specifications may be inputted manually into a Toolinput screen 2050 shown in FIG. 21. The cassette alignment tool systemmay also be programmed to provide these values automatically in responseto the operator inputting the bade type at an input box 2060 of theinput screen 2050.

[0175] The following is a calculation of the preferred slot base “N”height, where N=total number of slots and${{Reference}\quad {Dim}} = \begin{matrix}{{the}\quad {height}\quad {of}\quad {the}\quad {reference}\quad ({zero})\quad {surface}\quad 520} \\{\quad {{from}\quad {the}\quad {cassette}\quad {base}\quad {surface}\quad 220.\quad {This}\quad {is}}} \\{\quad {D_{NOTINV}\quad {in}\quad {the}\quad {illustrated}\quad {{embodiment}\quad.}}}\end{matrix}$

[0176] First, calculate a value for the variable Slot_Spacing, that is,the spacing from the center of one slot to the center of an adjacentslot: ${Slot\_ Spacing} = \frac{{Dist\_ Slot1} - {to} - {SlotN}}{N - 1}$

[0177] where Dist_Slot1_to_SlotN is the spacing from the center of slot1 to the center of slot N as shown in the Cassette Specifications inputscreen 1900 in FIG. 19. The value for Dist_Slot1-to_SlotN may beinputted at 1904 or provided automatically in response to the operatorinputting the model of the wafer cassette.

[0178] Calculate SlotN_center which is the height of the center of thetop slot N:

[0179] SlotN_center=(Dist_Slot1_to_SlotN)+(Dist_Base_to_Slot11)

[0180] where Dist_Base_to_Slot1 is the spacing from the platform baseplane 220 to the center of slot 1 as shown in FIG. 19. Again, the valuefor Dist_Base_to_Slot1 may be provided automatically by the system orinputted manually at 1906.

[0181] Calculate RootBaseN which is the base height of the root of thetop slot as indicated at 1908.

RootBaseN=(SlotN_center)−(RootHeight/2)

[0182] where the root height (RootHeight) may be inputted at 1910.

[0183] Calculate EffectiveToothLength, which is the effective length ofa tooth 1912 of the slot. This is the amount of the slot teeth 1912 perside that is outside the diameter of the wafer when it is resting in aslot.

EffectiveToothLength=((SlotRootWidth)−(WaferDiameter))/2

[0184] Calculate the drop height of a wafer underside below the root ofa slot when the wafer is resting upon the lower slot tooth surfaces.

WaferDropHeight=(sin(SlotToothAngle))*(EffectiveToothLength)

[0185] Calculate the height of the underside of a wafer resting in thetop slot.

WaferN_underside=(RootBaseN)−(WaferDropHeight)

[0186] Calculate the height of the underside of a wafer resting in thenext slot down from the top slot.

WaferN−1_underside=(WaferN_underside)−(Slot_Spacing)

[0187] Calculate the height of the topside of a wafer resting in thenext slot down from the top slot.

WaferN−1topside=(WaferN−1_underside)+(WaferThickness)

[0188] Determine the vertical center height offset of the space betweenthe underside of the topmost wafer and the topside of the next waferdown, both resting in their respective slots.

VSCenter=((WaferN_underside)−(WaferN−1_topside))/2

[0189] Calculate the height that the vertical center offset representsfrom the base of the metrology cassette 410.

VSHeight=(VSCenter)+(WaferN−1_topside)

[0190] Determine the maximum effective blade thickness. This is thelarger value of either the blade thickness (if the wafer is fullyreceived within the blade pocket and does not extend above the blade topsurface) or the sum of the blade pocket thickness plus the waferthickness (the wafer extends above the blade top surface).

[0191] Which ever one is larger:

WBThickness=(BladeThickness)

[0192] or:

WBThickness=((BladePocket)+(WaferThickness))

[0193] Calculate the mechanical center (vertical) of the effective blade(see above).

WBCenter=(WBThickness)/2

[0194] Determine the height of the lowest edge of the effective bladethickness when the blade is centered in the available space.

WB_underside=((VSHeight)−(WBCenter))

[0195] Determine the underside height of a wafer placed on the bladewhile centered in the available space between the two wafers.

Wafer_underside=((WB_underside)+(BladePocket))

[0196] Calculate the measurement that the laser sensors would “see”under these ideal conditions expressed as an offset distance to thereference surface 520. This is the preferred slot base measurement forthe topmost slot.

WaferN_SlotBase=((Re ferenceDim)−(Wafer_Underside))

[0197] Calculation of the preferred slot delta “N” height

[0198] Calculate the slot spacing.${Slot\_ Spacing} = \frac{{Dist\_ Slot1} - {to} - {SlotN}}{N - 1}$

[0199] Calculate the center of the top slot.

SlotN_center=(Dist_Slot1-to-slotN)+(Dist_Base-to-Slot1)

[0200] Determine the underside height of a wafer placed on the bladewhile centered in the topmost slot.

Wafer_underside=((slotN_center)−((WaferThickness)/2))

[0201] Calculate the measurement that the laser sensors would “see”under these ideal conditions expressed as an offset to the referencesurface 520. This is the preferred slot delta measurement for thetopmost slot.

WaferN_SlotDelta=((Re ferenceDim)−(Wafer_Underside))

[0202] In accordance with another aspect of the present embodiments, anadditional consideration when calculating preferred heights of wafersresting in actual cassette slots is the curvature of the workpiece edge2300 (FIG. 23) of a wafer 230 b. This curvature serves to reduceoperator injury and to reduce stress cracking of the workpiece byrelieving stresses.

[0203] As shown in FIG. 23, the effect of the edge curvature 2300 is tophysically lower a workpiece such as the workpiece 230 b for example, ascompared to a workpiece 230 c (shown in phantom) having a flat edge,relative to the platform reference surface 220 (FIG. 5) of the handlerplatform 200. The amount that the curvature causes the workpiece to dropwithin the slot may be calculated as a function of the curvature ofworkpiece edge and the shape of the slot of the cassette. The curvature2300 may follow a well-defined industry standard ( such as SEMI M1-0298)or may have a proprietary curvature. In the illustrated embodiment, theheight differential WaferEdgeDrop by which the workpiece is lowered maybe calculated by using the following formula which is of sufficientaccuracy for many applications having customary values of the slot toothangle and wafer curvature:

WaferEdgeDrop=(sinh(SlotToothAngle)*W_(THICK))

[0204] (where sinh represents the hyperbolic sine of the referencedangle and W_(THICK) is the thickness of the wafer).

[0205] The resulting WaferEdgeDrop distance may be used to adjust thepredicted height of wafers resting in cassette wafer slots downward. Itshould be appreciated that other wafer edge profiles can also bemathematically simulated and that they may also be used to predict theamount of vertical displacement of a wafer workpiece in a cassette slot.

[0206] Yet another calculation is the amount of front-to-rear verticaldisplacement that may result from the wafer workpiece 230 being seatedcompletely inward within cassette wafer slots 204 as shown in FIGS. 24aand 24 b. This front-to-rear vertical displacement is typically causedby the wafer being supported at an angle by the sloped surface of a slottooth 1912 primarily at the inward (rearmost) edge 2400 of the wafer. Asa result, the wafer 230 is pivoted about an axis formed by a chordacross the workpiece defined by the left and right side edges, andperpendicular in the horizontal plane to the normal insertion andwithdrawal motion for the workpiece relative to the cassette.

[0207] The effect of this displacement is to reduce the availablevertical motion space that is available for the robot blade 206 by anamount equal to the total of the displacement and which is dividedequally on the top and bottom of the otherwise calculated verticalmotion space.

[0208] The displacement InducedDroop may be calculated using thefollowing formula:

InducedDroop=(WaferDropHeight*2)

[0209] Like the wafer edge drop calculation, the calculated induceddroop distance may be used to adjust the predicted height of wafersresting in cassette wafer slots downward.

[0210] It will be recognized of course that other preferred heights maybe calculated or otherwise determined to provide a basis for comparisonto measured height values.

[0211] Interface Controller 412 Construction and Features

[0212] The interface controller 412 of the illustrated embodiment servesmultiple functions in the cassette alignment tool system 400 set. Amongother things, it acts as a power conditioning and distribution center, asignal conditioner and converter, display, communications driver, andoperator interface. Thus, the computer generated graphical interface maybe eliminated in some applications.

[0213] Outputs from the laser sensors range from −5.0000 to +5.0000volts. This voltage range corresponds to the limits of the linearmeasurement range, as previously mentioned. An internal high-precisionanalog-to-digital converter 2110 (FIG. 22) is used to change theincoming voltage level into a signed binary number which is thenconverted to Inch or Metric readings for display or transmission. Thedisplay conversion range in the illustrated embodiment is −1.5745″(−40,00 mm) to +1.5745″ (+40,00 mm), which represents an input voltagerange of −10.0000 to +10.0000 volts. Because the sensors of theillustrated embodiment output half this voltage range, representing halfthis distance, the usable display range is −0.7875″ (−20,00 mm) to+0.7875″ (+20,00 mm).

[0214] Due to the highly sensitive nature of the sensors preferably usedin this tool, their outputs can be relatively noisy, having electricalglitches and “shot” noise superimposed on their output signals. This ispreferably carefully filtered out by a filter circuit 2120 which couplesthe signal line of the cable 414 from the laser sensors to an analoginput multiplexer 2130. An isolation amplifier 2135 isolates themultiplexer output from the converter input. In addition, the outputs ofthe laser heads are referenced to an analog ground point AR which iscoupled to ground point 2140 inside the sensors' amplifiers 2150 by apower return line 2152 which has an effective line wiring resistance asrepresented by resistance 2154. For this reason, the outputs also carrya common mode voltage offset voltage component in addition to everythingelse.

[0215] As shown in the filter circuit 2120, the cassette alignment toolsystem 400 interface controller 412 is specially designed to filter theincoming signals relative to their internal ground points including thecable shielding 2160, while also filtering the ground point voltages.These are sensed and digitized separately and compared to determine thetrue signal voltage outputs from the sensors.

[0216] Despite all the analog filtering, local environment RFI (radiofrequency interference), low frequency AC fields, and magnetic fieldscan still affect readings. To reduce or eliminate the effects of theseenvironmental factors, the sensors are preferably sampled many times andthe results are averaged to obtain the readings that are finallydisplayed. An options switch on the bottom of the PC board in theinterface controller 412 can control how many readings are averaged.

[0217] Once filtered, converted, sampled, averaged, the readings aredisplayed on the local LCD screen 530 and are also broadcast on theserial port to the computer 416.

[0218] Information transmitted on the serial port is updated once persecond typically. In addition, the driver software for the serial portemits a synchronization signal and senses for a similar signal from aremote connection. The transmitted signal is used to indicate to thecassette alignment tool system 400 that an interface controller 412 isconnected and active. When a similar signal is received from thecassette alignment tool system 400 (or other host), the interfacecontroller 412 switches from local to remote mode. In this mode, the LCDdisplay is not updated periodically. Instead, it serves as a dataterminal display for the cassette alignment tool system 400, allowingmessages to be sent and shown.

[0219] Other than during the warmup period, the front panel buttons arepreferably continuously scanned. Activation of any of these buttonscauses a message to be sent over the serial port. The inch/metric switchcondition is preferably not transmitted to the computer 416 of thecassette alignment tool system 400, as it contains its own optionselector for this condition.

[0220] The interface controller 412 is a metal clamshell structure withthe majority of the electronics attached to its front face cover. Thelaser sensor amplifiers are mounted to its base. Multicolorsilkscreening and grouped connectors help to prevent connection errors.The extender cords for the laser measurement heads are also color coded.The interface controller 412 accommodates five laser sensors, althoughthree are shown installed in the illustrated embodiment. More or fewersensors may be provided depending upon the applications. These sensorsare color coded and correspond directly to the red, blue and yellowcolor-coded laser sensor heads on the metrology cassette 410.

[0221] A 4 line by 40 character high contrast LCD display with backlight is provided. Indicator LEDs for the slide switch-selectableEnglish/Metric mode display and for prompting the operator duringprocedures are available on the front face. An RS-232 serial portenables connection and communication to the cassette alignment toolsystem computer 416. This connection provides ASCII (human readable)data in a 9600,N,8,1 format. Connections to standard DB-9M PC COM ports(IBM-AT standard) are accomplished using a 9 wire male-female passthrough cable. A null modem adapter or cable is preferably not to beused for normal connection to standard PC ports. Front panel pushbuttonsinclude Zero, Back, Select, and Next functions. When communications areestablished with a host computer and the cassette alignment tool system400, the functions of these buttons are echoed in the cassette alignmenttool system 400. The corded “universal” switching power supply accepts90-265 VAC inputs from 45-75 hertz. The power supply accepts worldstandard IEC320 style line cords which allow the operator to plug inwhatever local style is appropriate. Alternatively the interfacecontroller 412 will accept “clean” 24+/−4 VDC from any convenientsource. The center pin of the rear-panel-mounted power jack is positive.The power input is reverse polarity protected and fused. The power tothe laser heads is preferably not provided to all of the heads at onetime at start up. Instead, it is preferred that the heads be switched onin sequence, one at a time to facilitate proper operation.

[0222] A single 16-bit analog-to-digital converter is utilized forconversions from the laser head outputs to numeric information. Thispromotes uniformity and stability. The laser head signal inputs areheavily electrically filtered to enhance rejection of electrical and RFnoise, as well as to reduce the effects of “shot noise” in theirsignals. The analog multiplexing circuitry is buffered to minimizevariations from channel-to-channel. Multiplexer-induced variations aretypically less than 0.002% of the final readings, therefore they arenegligible. Samples of the laser head outputs are taken 160 times persecond, but 128 or 256 readings are averaged to obtain each updatevalue. This provides improved immunity to false readings caused by ACline pickup and line noise. The signals are taken from the laser headsin “Kelvin” style. That is to say that the ground reference is takenfrom a separate connection that is referenced intemally to the laserheads. The true signals are differential voltages from this referencepoint. This technique reduces or eliminates “ground loop” (common mode)voltage effects. The options switch on the printed circuit board allowsthe operator to select the displayed resolution for English (inch)measurements. If set to “off”, inch measurements are shown on the box'sdisplay as four decimal place numbers. If “on” they are displayed asthree decimal place numbers. The switch setting has no effect uponmetric (mm) displays, but has one other effect. When “off”, 256measurement samples are averaged to obtain each display update. When“on”, 128 measurement samples are averaged. This affects the displayupdate and reporting speed, but provides significantly greater stabilityfor four decimal place displays.

[0223] The dimensions, ranges, shapes, materials, sizes,characteristics, finishes, processes and values of the interfacecontroller construction and circuitry are provided as examples and canvary, depending upon the intended application.

[0224] It will, of course, be understood that modifications of theillustrated embodiments, in its various aspects, will be apparent tothose skilled in the art, some being apparent only after study othersbeing matters of routine mechanical and electronic design. Otherembodiments are also possible, their specific designs depending upon theparticular application. For example, a variety of methods and devicesfor physical measurements may be utilized in addition to those describedabove. Such methods and devices may include, for example, inductive andcapacitive proximity sensors, non-laser optical sensors, sonic distancesensors and others. A variety of workpiece cassette shapes and sizes mayalso be utilized. Furthermore, a variety of graphical displays visuallydepicting alignments and misalignments and the degrees of such may beused in addition to or instead of numerical displays. As such, the scopeof the invention should not be limited by the particular embodimentsdescribed herein but should be defined by the appended claims andequivalents thereof.

What is claimed is:
 1. An alignment tool for aligning a cassette handlerto a robot blade in a workpiece handling system wherein said cassettehandler has a support surface for supporting a workpiece cassette havinga plurality of slots for carrying workpieces, comprising: a frameadapted to be supported by said cassette handler support surface in afirst orientation of said frame relative to said handler supportsurface, said frame having a reference surface and a first supportsurface positioned to engage said handler support surface and to supportsaid reference surface a first predetermined distance from said handlersupport surface in said first orientation of said frame relative to saidhandler support surface; and a first distance sensor positioned tomeasure the distance of said reference surface from said sensor and tomeasure the distance of a workpiece from said sensor.
 2. The alignmenttool of claim 1 wherein said workpiece cassette has registrationsurfaces and said cassette handler has registration surfaces adapted tomate with said cassette registration surfaces to register said cassetteto said handler, said frame having first registration surfaces adaptedto mate with said handler registration surfaces and to register saidframe to said handler in said first orientation of said frame.
 3. Thealignment tool of claim 1 wherein said frame is adapted to be supportedby said cassette handler support surface in a second orientation of saidframe relative to said handler support surface ,said frame having asecond support surface positioned to engage said handler support surfaceand to support said reference surface a second predetermined distancefrom said handler support surface in said second orientation of saidframe relative to said handler support surface.
 4. The alignment tool ofclaim 3 wherein said frame has second registration surfaces adapted tomate with said handler registration surfaces and to register said frameto said handler in said second orientation of said frame.
 5. Thealignment tool of claim 1 wherein said distance sensor is carried withinsaid frame.
 6. The alignment tool of claim 1 wherein said distancesensor includes a laser head.
 7. The alignment tool of claim 1 whereinsaid first distance sensor is positioned to measure the distance of afirst location of said reference surface from said sensor and to measurethe distance of a first location of said workpiece from said sensor,said tool further comprising a second distance sensor positioned tomeasure the distance of a second location of said reference surface fromsaid second sensor and to measure the distance of a second location ofsaid workpiece from said second sensor.
 8. The alignment tool of claim 7wherein said cassette handler has a mechanism for adjusting theorientation of said platform along a first direction, wherein said firstand second sensors are disposed along a line parallel to said firstdirection.
 9. The alignment tool of claim 8 further comprising a thirddistance sensor positioned to measure the distance of a third locationof said reference surface from said third sensor and to measure thedistance of a third location of said workpiece from said third sensor.10. The alignment tool of claim 9 wherein said cassette handler has amechanism for adjusting the orientation of said platform along a seconddirection, wherein said second and third sensors are disposed along aline parallel to said second direction.
 11. An alignment tool foraligning a cassette handler to a robot blade in a workpiece handlingsystem for handling a workpiece cassette having a plurality of slots forcarrying workpieces wherein said cassette handler has a support surfacefor supporting said cassette and registration surfaces adapted to matewith cassette registration surfaces to register said cassette to saidhandler, comprising: a frame adapted to be supported by said cassettehandler support surface in a first orientation of said frame relative tosaid handler support surface, said frame having first registrationsurfaces adapted to mate with said handler registration surfaces and toregister said frame to said handler in said first orientation of saidframe; and a a distance sensor positioned to measure the distance ofsaid reference surface from said sensor and to measure the distance of aworkpiece from said sensor.
 12. An alignment tool for aligning acassette handler to a robot blade in a workpiece handling system forhandling a workpiece cassette having a plurality of slots for carryingworkpieces wherein said cassette handler has a support surface forsupporting said cassette and registration surfaces adapted to mate withcassette registration surfaces to register said cassette to saidhandler, comprising: a frame adapted to be supported by said cassettehandler support surface; and a plurality of distance sensors positionedwithin said frame to measure the orientation of a predetermined planewithin said frame relative to a workpiece carried by said robot blade.13. The tool of claim 12 wherein said predetermined frame plane is at aposition which corresponds to a slot base position.
 14. The tool ofclaim 12 wherein said predetermined orientation of said predeterminedframe plane is substantially parallel to said workpiece carried by saidrobot blade.
 15. The tool of claim 12 wherein said plurality of distancesensors are positioned to measure the distances of a plurality oflocations on a workpiece carried by said robot blade relative to saidpredetermined frame plane.
 16. The tool of claim 15 wherein each of saidsensors comprises a laser distance sensor carried by said frame andpositioned to measure one of said distances of said plurality oflocations on said workpiece carried by said robot blade relative to saidpredetermined frame plane.
 17. The tool of claim 15 further comprising adisplay for displaying a numerical representation of each of saidmeasured distances of said plurality of locations on said workpiececarried by said robot blade relative to said predetermined frame plane.18. The tool of claim 12 further comprising a display for displaying atleast one of a numerical representation and a graphical representationof said measured orientation of said predetermined frame plane relativeto said workpiece carried by said robot blade.
 19. The tool of claim 15further comprising a calculator for calculating a difference betweenselected measured distances of selected locations of said plurality oflocations on said workpiece carried by said robot blade relative to saidpredetermined frame plane.
 20. The tool of claim 19 further comprising adisplay for displaying a numerical representation of said calculateddifference between selected measured distances of selected locations ofsaid plurality of locations on said workpiece carried by said robotblade relative to said predetermined frame plane.
 21. A method ofaligning a cassette handler to a robot blade in a workpiece handlingsystem wherein said cassette handler has a support surface forsupporting a workpiece cassette having a plurality of slots for carryingworkpieces, comprising: placing a frame on said cassette handler supportsurface in a first orientation of said frame relative to said handlersupport surface; and determining the distance of a workpiece carried bysaid robot blade relative to a predetermined reference surface carriedby said frame.
 22. The method of claim 21 further comprising comparing apredetermined distance from said reference surface to said determineddistance of said workpiece relative to said predetermined referencesurface and changing an offset value of said cassette handler as afunction of said comparison.
 23. The method of claim 22 wherein saidpredetermined distance corresponds to a slot base height defined by saidworkpiece cassette.
 24. The method of claim 22 wherein saidpredetermined distance corresponds to a slot delta height defined bysaid workpiece cassette.
 25. The method of claim 21 further comprisingplacing said frame on said cassette handler support surface in a secondorientation of said frame relative to said handler support surface; anddetermining the distance of a workpiece carried by said robot bladerelative to said predetermined reference surface carried by said framein said second orientation.
 26. The method of claim 21 furthercomprising calibrating a distance measuring sensor carried by saidframe, said calibrating including measuring the distance between saidsensor and said reference surface to provide a reference distance,wherein said workpiece distance measuring comprises measuring thedistance between said sensor and said workpiece to provide a workpiecedistance and calculating the difference between said workpiece distanceand said reference distance to provide an offset distance of saidworkpiece relative to said predetermined reference surface.
 27. A methodof aligning a cassette handler to a robot blade in a workpiece handlingsystem wherein said cassette handler has a support surface forsupporting a workpiece cassette having a plurality of slots for carryingworkpieces, comprising: measuring at a first height of said robot blade,a first distance of a workpiece carried by said robot blade relative toa predetermined reference point; determining at said first height ofsaid robot blade, a first step count of an elevator for elevating saidframe relative to said robot blade; measuring at a second height of saidrobot blade, a second distance of said workpiece carried by said robotblade relative to said predetermined reference point; determining atsaid second height of said robot blade, a second step count of saidelevator; and calculating a pitch of said elevator as a function of thedifference between said measured first and second distances and thedifference between said first and second counts.
 28. The method of claim27 wherein said predetermined reference is defined by said handlersupport surface.
 29. The method of claim 27 wherein said first heightmeasuring includes placing a frame on said cassette handler supportsurface in a first orientation of said frame relative to said handlersupport surface; and determining the distance of said workpiece carriedby said robot blade relative to a predetermined reference surfacecarried by said frame.
 30. The method of claim 29 wherein said secondheight measuring includes u placing said frame on said cassette handlersupport surface in a second orientation of said frame relative to saidhandler support surface; and determining the distance of said workpiececarried by said robot blade relative to said predetermined referencesurface carried by said frame in said second orientation.
 31. A systemof calibrating a cassette handler to a robot blade in a workpiecehandling system wherein said cassette handler has a support surface forsupporting a workpiece cassette having a plurality of slots for carryingworkpieces and an elevator for elevating said support surface a distancedefined in steps, comprising: a distance sensor positioned to measurethe height of a workpiece carried by said robot blade; and a processorhaving a memory for storing a first step count of said elevator at afirst height of a workpiece carried by said robot blade; a firstmeasured height of said workpiece at said first height; a second stepcount of said elevator at a second height of said workpiece carried bysaid robot blade; and a second measured height of said workpiece at saidsecond height; said processor for calculating a pitch of said elevatoras a function of the difference between said measured first and secondheights and the difference between said first and second counts.
 32. Thesystem of claim 31 wherein said measured height is measured relative tosaid handler support surface.
 33. The system of claim 31 furthercomprises a frame adapted to carry said distance sensor and having areference surface spaced from said sensor to provide a referencedistance wherein the height of a workpiece may be measured relative tosaid reference surface of said frame.
 34. The system of claim 33 whereinsaid frame has a first registration surface adapted to mate with saidhandler support surface in a first orientation of said frame, and asecond registration surface adapted to mate with said handler supportsurface in a second orientation of said frame which is inverted withrespect to said first orientation.
 35. A method of aligning a cassettehandler to a robot blade in a workpiece handling system wherein saidcassette handler has a support surface for supporting a workpiececassette having a plurality of slots for carrying workpieces,comprising: placing a frame on said cassette handler support surface;moving a workpiece carried by said robot blade; and mapping the motionof said workpiece carried by said robot blade relative to said frame.36. The method of claim 35 further comprising comparing said mappedmotion to predetermined limits of travel of said workpiece relative tosaid frame.
 37. The method of claim 36 further comprising providing anout-of-bounds indication if a portion of said mapped motion exceeds apredetermined limit of travel.
 38. The method of claim 35 furthercomprising displaying said mapped motion.
 39. The method of claim 38further comprising comparing said mapped motion to predetermined limitsof travel of said workpiece relative to said frame and displaying. 40.The method of claim 39 further comprising displaying an out-of-boundsindication if a portion of said mapped motion exceeds a predeterminedlimit of travel.
 41. The method of claim 39 further comprisingdisplaying said predetermined limit of travel overlayed on said displayof said mapped motion.
 42. An alignment tool kit for aligning a cassettehandler to a movable robot blade for carrying a workpiece in a workpiecehandling system wherein said cassette handler has a support surface forsupporting a workpiece cassette having a plurality of slots for carryingworkpieces, comprising: an alignment member; and a frame adapted to besupported by said cassette handler support surface in a firstorientation of said frame relative to said handler support surface, saidframe having an alignment surface for receiving said alignment member,said alignment surface defining a first predetermined robot bladeposition; wherein said robot blade has an alignment surface forreceiving said alignment member wherein said robot blade is positionedin said predetermined position when said member is received by saidalignment surfaces of said robot blade and frame.
 43. The tool kit of aclaim 42 wherein alignment member comprises a pin and said frame androbot blade alignment surfaces each define an aperture shaped to receivesaid alignment pin.
 44. The tool kit of a claim 42 wherein saidworkpiece handling system includes a robot for rotating and extendingsaid robot blade, said predetermined robot blade position comprising arobot blade extension position and a robot blade rotation position. 45.The tool kit of claim 44 wherein said first predetermined robot bladeposition is a robot blade workpiece drop-off position.
 46. The tool kitof claim 45 wherein said first predetermined robot blade position is ata first height of said blade relative to said handler support surface,said first height corresponding to the height of said blade at a slotbase position.
 47. The alignment tool kit of claim 42 wherein said firstpredetermined robot blade position is at a first height of said bladerelative to said handler support surface, said frame being adapted to besupported by said cassette handler support surface in a secondorientation of said frame relative to said handler support surface, saidalignment surface defining a second predetermined robot blade positionwhen said frame is in said second orientation; wherein said robot bladeis positioned in said second predetermined position when said member isreceived by said alignment surface of said robot blade and by said framealignment surface in said second orientation.
 48. The tool kit of aclaim 47 wherein said workpiece handling system includes a robot forrotating and extending said robot blade, said second predetermined robotblade position comprising a robot blade extension position and a robotblade rotation position.
 49. The tool kit of claim 47 wherein saidsecond predetermined robot blade position is a robot blade workpiecedrop-off position.
 50. The tool kit of claim 47 wherein said secondpredetermined robot blade position is at a second height of said bladerelative to said handler support surface, said second heightcorresponding to the height of said blade at a slot base position.
 51. Amethod of aligning a cassette handler to a robot blade in a workpiecehandling system wherein said cassette handler has a support surface forsupporting a workpiece cassette having a plurality of slots for carryingworkpieces, comprising: placing a frame on said cassette handler supportsurface in a first orientation of said frame relative to said handlersupport surface; measuring the orientation of a predetermined referencesurface carried by said frame relative to a workpiece carried by saidrobot blade; and adjusting the orientation of said cassette handlersurface so that said predetermined reference surface of said frame has apredetermined orientation relative to said workpiece carried by saidrobot blade.
 52. The method of claim 51 wherein said predeterminedorientation of said predetermined reference surface is substantiallyparallel to said workpiece carried by said robot blade.
 53. The methodof claim 51 wherein said measuring comprises measuring the distances ofa plurality of locations on a workpiece carried by said robot bladerelative to said predetermined reference surface carried by said frame.54. The method of claim 51 wherein said distances measuring comprisesusing laser distance sensors carried by said frame and positioned tomeasure said distances of said plurality of locations on said workpiececarried by said robot blade relative to said predetermined referencesurface carried by said frame.
 55. The method of claim 54 furthercomprising displaying a numerical representation of each of saidmeasured distances of said plurality of locations on said workpiececarried by said robot blade relative to said predetermined referencesurface carried by said frame.
 56. The method of claim 51 furthercomprising displaying at least one of a numerical representation and agraphical representation of said measured orientation of saidpredetermined reference surface carried by said frame relative to saidworkpiece carried by said robot blade.
 57. The method of claim 56wherein said adjusting comprises adjusting the orientation of saidcassette handler surface until said displayed representation of saidmeasured orientation corresponds to said predetermined orientation ofsaid predetermined reference surface of said frame relative to saidworkpiece carried by said robot blade.
 58. A method of aligning acassette handler to a robot blade in a workpiece handling system whereinsaid cassette handler has a support surface for supporting a workpiececassette having a plurality of slots for carrying workpieces,comprising: placing a frame emulating a workpiece cassette on saidcassette handler support surface in a first orientation of said framerelative to said handler support surface; measuring the orientation of apredetermined plane within said frame relative to a workpiece carried bysaid robot blade; and adjusting the orientation of said cassette handlersurface so that said predetermined frame plane has a predeterminedorientation relative to said workpiece carried by said robot blade. 59.The method of claim 58 wherein said predetermined frame plane is at aposition which corresponds to a slot base position.
 60. The method ofclaim 58 wherein said predetermined orientation of said predeterminedframe plane is substantially parallel to said workpiece carried by saidrobot blade.
 61. The method of claim 58 wherein said measuring comprisesmeasuring the distances of a plurality of locations on a workpiececarried by said robot blade relative to said predetermined frame plane.62. The method of claim 58 wherein said distances measuring comprisesusing laser distance sensors carried by said frame and positioned tomeasure said distances of said plurality of locations on said workpiececarried by said robot blade relative to said predetermined frame plane.63. The method of claim 62 further comprising displaying a numericalrepresentation of each of said measured distances of said plurality oflocations on said workpiece carried by said robot blade relative to saidpredetermined frame plane.
 64. The method of claim 63 wherein saidadjusting comprises adjusting the orientation of said cassette handlersurface until each of said displayed numerical representations of eachof said measured distances of said plurality of locations on saidworkpiece carried by said robot blade relative to said predeterminedframe plane is substantially equal to zero.
 65. The method of claim 58further comprising displaying at least one of a numerical representationand a graphical representation of said measured orientation of saidpredetermined frame plane relative to said workpiece carried by saidrobot blade.
 66. The method of claim 65 wherein said adjusting comprisesadjusting the orientation of said cassette handler surface until saiddisplayed representation of said measured orientation corresponds tosaid predetermined orientation of said predetermined frame planerelative to said workpiece carried by said robot blade.
 67. A method ofaligning a cassette handler to a robot blade in a workpiece handlingsystem wherein said cassette handler has a support surface forsupporting a workpiece cassette having a plurality of slots for carryingworkpieces, comprising: placing a frame emulating a workpiece cassetteon said cassette handler support surface in a first orientation of saidframe relative to said handler support surface; measuring the height ofa workpiece carried by said robot blade relative to a predeterminedplane within said frame; and adjusting the height of said cassettehandler surface so that workpiece carried by said robot blade issubstantially at the height of said predetermined frame plane.
 68. Themethod of claim 67 wherein said predetermined frame plane is at aposition which corresponds to a slot base position.
 69. The method ofclaim 67 wherein said predetermined frame plane is at a position whichcorresponds to a slot delta position.
 70. The method of claim 67 whereinsaid measuring comprises measuring the distances of a plurality oflocations on a workpiece carried by said robot blade relative to saidpredetermined frame plane.
 71. The method of claim 70 wherein saiddistances measuring comprises using laser distance sensors carried bysaid frame and positioned to measure said distances of said plurality oflocations on said workpiece carried by said robot blade relative to saidpredetermined frame plane.
 72. The method of claim 71 further comprisingdisplaying a numerical representation of each of said measured distancesof said plurality of locations on said workpiece carried by said robotblade relative to said predetermined frame plane.
 73. The method ofclaim 72 wherein said predetermined frame plane is at a position whichcorresponds to a slot delta position and said adjusting comprisesadjusting a bottom slot offset count of an elevator for elevating saidcassette handler support surface until each of said displayed numericalrepresentations of each of said measured distances of said plurality oflocations on said workpiece carried by said robot blade relative to saidslot base plane is substantially equal to zero.
 74. The method of claim72 wherein said predetermined frame plane is at a position whichcorresponds to a slot delta position and said adjusting comprisesadjusting a pickup slot offset count of an elevator for elevating saidcassette handler support surface until each of said displayed numericalrepresentations of each of said measured distances of said plurality oflocations on said workpiece carried by said robot blade relative to saidslot delta plane is substantially equal to zero.
 75. The method of claim67 further comprising displaying at least one of a numericalrepresentation and a graphical representation of each of said measureddistances of said plurality of locations on said workpiece carried bysaid robot blade relative to said predetermined frame plane.
 76. Themethod of claim 75 wherein said adjusting comprises adjusting a bottomslot offset count of an elevator for elevating said cassette handlersupport surface until said displayed representations of each of saidmeasured distances of said plurality of locations on said workpiececarried by said robot blade relative to said predetermined frame planeare each substantially equal to zero.
 77. The method of claim 75 whereinsaid adjusting comprises adjusting a pickup slot offset count of anelevator for elevating said cassette handler support surface until saiddisplayed representations of each of said measured distances of saidplurality of locations on said workpiece carried by said robot bladerelative to said predetermined frame plane are each substantially equalto zero.
 78. A method of aligning a cassette handler to a robot blade ina workpiece handling system wherein said cassette handler has a supportsurface for supporting a workpiece cassette having a plurality of slotsfor carrying workpieces, comprising: placing a frame on said cassettehandler support surface in a first orientation of said frame relative tosaid handler support surface, said frame having a distance sensor;placing a workpiece on said frame at a predetermined distance from saidsensor; measuring the distance of said workpiece from said sensor usingsaid sensor; relative to a predetermined reference surface carried bysaid frame; and comparing said measured distance to said predetermineddistance to provide a correction factor for correcting distancemeasurements to said workpiece.
 79. The method of claim 78 wherein saidframe has a reference surface, and wherein said workpiece placingcomprises placing said workpiece on said frame reference surface andsaid predetermined distance is determined using said sensor prior toplacing said workpiece on said frame reference surface.
 80. An alignmenttool for aligning a cassette handler to a robot blade in a workpiecehandling system wherein said cassette handler has a support surface forsupporting a workpiece cassette having a plurality of slots for carryingworkpieces, comprising: a frame adapted to be supported by said cassettehandler support surface; a distance sensor positioned within said frameto sense, relative to said frame, the position of a workpiece carried bysaid robot blade; a display for displaying a graphical representation ofthe sensed position of said workpiece.
 81. The tool of claim 80 furthercomprising a comparator for comparing said sensed position to apredetermined limit of travel of said workpiece relative to said frame.82. The tool of claim 81 wherein said display displays an out-of-boundsindication if a sensed position of said workpiece exceeds apredetermined limit of travel.
 83. The tool of claim 80 wherein saidsensed position is in a vertical direction, and wherein said display isresponsive to the horizontal position of said robot blade carrying saidworkpiece, to display a graphical representation of a plurality ofpositions of the workpiece to define a path of motion, each workpieceposition of the path being represented as a function of both said sensedvertical position of said workpiece and said robot blade horizontalposition.
 84. The tool of claim 83 wherein said display includes acomputer display screen.
 85. The tool of claim 83 further comprising acomparator for comparing said plurality of positions to a predeterminedlimit of travel of said workpiece relative to said frame, wherein saiddisplay displays an out-of-bounds indication if a sensed position ofsaid workpiece exceeds a predetermined limit of travel.
 86. The tool ofclaim 85 wherein said display further displays said predetermined limitof travel overlayed on said display of said plurality of workpiecepositions.
 87. A method of aligning a cassette handler to a movablerobot blade for carrying a workpiece in a workpiece handling systemwherein said cassette handler has a support surface for supporting aworkpiece cassette having a plurality of slots for carrying workpieces,comprising: placing a frame on said cassette handler support surface ina first orientation of said frame relative to said handler supportsurface, said frame having an alignment surface aligned with a firstpredetermined robot blade position, said frame alignment surface beingadapted to receive an alignment member; positioning said robot blade insaid frame wherein said robot blade has an alignment surface adapted toreceive said alignment member; and testing the alignment of said robotblade relative to said frame, said testing including placing saidalignment member in engagement with at least one of said alignmentsurfaces of said frame and said blade; wherein said robot blade ispositioned in said predetermined position when said alignment member isreceived by said alignment surfaces of both said robot blade and frame.88. The method of a claim 87 wherein said alignment member comprises apin and said frame and robot blade alignment surfaces each define anaperture shaped to receive said alignment pin.
 89. The method of a claim87 wherein said workpiece handling system includes a robot for rotatingand extending said robot blade, said predetermined robot blade positioncomprising a robot blade extension position and a robot blade rotationposition.
 90. The method of claim 89 wherein said first predeterminedrobot blade position is a robot blade workpiece drop-off position. 91.The method of claim 90 wherein said first predetermined robot bladeposition is at a first height of said blade relative to said handlersupport surface, said first height corresponding to the height of saidblade at a slot base position.
 92. The method of claim 87 wherein saidfirst predetermined robot blade position is at a first height of saidblade relative to said handler support surface, said method furthercomprising: placing said frame on said cassette handler support surfacein a second orientation of said frame relative to said handler supportsurface; positioning said robot blade in said frame in a secondpredetermined robot blade position at a second height of said bladerelative to said handler support surface wherein said robot blade has analignment surface adapted to receive said alignment member; and testingthe alignment of said robot blade relative to said frame, said testingincluding placing said alignment member in engagement with at least oneof said alignment surfaces of said frame and said blade; wherein saidrobot blade is positioned in said predetermined position at said secondheight when said alignment member is received by said alignment surfacesof both said robot blade and frame in said second orientation.
 93. Themethod of a claim 92 wherein said positioning includes using a robot forrotating and extending said robot blade, said second predetermined robotblade position comprising a robot blade extension position and a robotblade rotation position.
 94. The method of claim 92 wherein said secondpredetermined robot blade position is a robot blade workpiece drop-offposition.
 95. The method of claim 92 wherein said second robot bladeheight corresponds to the height of said blade at a slot base position.96. A tool for aligning a cassette handler to a robot blade in aworkpiece handling system wherein said cassette handler has a supportsurface for supporting a workpiece cassette having a plurality of slotsfor carrying workpieces, comprising: a frame adapted to be supported onsaid cassette handler support surface in a first orientation of saidframe relative to said handler support surface, said frame having apredetermined reference surface adapted to carry a workpiece having aknown thickness, said frame further having a distance sensor positionedto measure the distance from said sensor to said reference surfaceabsent said workpiece and to measure the distance from said sensor tosaid workpiece carried by said reference surface; and a processorresponsive to said frame distance sensor for comparing the distancemeasured by said sensor to said predetermined reference surface absentsaid workpiece, to the distance measured by said sensor to saidworkpiece carried by said reference surface.
 97. The tool of claim 96wherein said processor computes a correction factor for correctingdistance measurements to said workpiece, said correction factor being afunction of said known thickness of said workpiece and the differencebetween said measured distance from said sensor to said referencesurface absent said workpiece and said measured distance from saidsensor to said workpiece carried by said reference surface.
 98. Analignment tool for aligning a cassette handler to a robot blade in aworkpiece handling system for handling a workpiece cassette having aplurality of slots for carrying workpieces wherein said cassette handlerhas a support surface for supporting said cassette and registrationsurfaces adapted to mate with cassette registration surfaces to registersaid cassette to said handler, comprising: a distance sensor positionedto measure the height of a workpiece carried by said robot blade,relative to said handler support surface; and a processor responsive tosaid sensor, for calculating a predetermined height and for comparingthe measured height of said workpiece to said predetermined height. 99.The tool of claim 98 wherein said processor has an input for receivingparameters for calculating said predetermined height, said inputparameters including dimensions of said wafer cassette.
 100. The tool ofclaim 99 wherein said wafer cassette dimensions include the spacing fromthe center of one slot to the center of an adjacent slot.
 101. The toolof claim 99 wherein said wafer cassette dimensions include the length ofat least one particular portion of a slot.
 102. The tool of claim 99wherein said wafer cassette parameters include the shape of at least oneparticular portion of a slot.
 103. The tool of claim 99 wherein saidinput parameters include dimensions of said workpiece.
 104. The tool ofclaim 103 wherein said predetermined height calculation is a function ofthe shape of the edge of the workpiece.
 105. The tool of claim 103wherein said predetermined height calculation is a function of the tiltof the workpiece in said workpiece cassette.
 106. The tool of claim 99wherein said input parameters include dimensions of said robot blade.